METHODS OF TREATMENT USING ANTI-ErbB ANTIBODY-MAYTANSINOID CONJUGATES

Abstract: The application concerns methods of treatment using anti-ErbB receptor antibody-maytansinoid conjugates, and articles of manufacture suitable for use in such methods. In particular, the invention concerns ErbB receptor-directed cancer therapies, using anti-ErbB receptor antibody-maytansinoid conjugates.

Description: METHODS OF TREATMENT USING ANTI-ErbB ANTIBODY-MAYTANSINOID CONJUGATES Background of the Invention Field of the Invention The present invention concerns methods of treatment, especially ErbB receptor-directed cancer therapies, using anti-ErbB receptor antibody-maytansinoid conjugates, and articles of manufacture suitable for use in such methods. Description of the Related Art 1. Maytansiπe and mavtansinoids Maytansine was first isolated from the east African shrub Maytenus serrata (U.S. Patent No. 3,896,111 ). Subsequently, it was discovered that certain microbes also produce maytansinoids, such as maytaπsinol and C-3 maytansinol esters (U.S. Patent No. 4,151,042). Synthetic maytansinol and maγtansinol analogues are disclosed, for example, in U.S. Patent Nos. 4,137,230; 4,248,870; 4,256,746; 4,260,608; 4,265,814; 4,294,757; 4,307,016; 4,308,268; 4,308,269; 4,309,428; 4,313,946; 4,315,929; 4,317,821; 4,322,348; 4,331,598; 4,361,650; 4,364,866; 4,424,219; 4,450,254; 4,362,663; and 4,371,533, the disclosures of which are hereby expressly incorporated by reference. Maytansine and maytansinoids are highly cytotoxic but their clinical use in cancer therapy has been greatly limited by their severe systemic side-effects primarily attributed to their poor selectivity for tumors. Clinical trials with maytansine had been discontinued due to serious adverse effects on the central nervous system and gastrointestinal system (Issel et al., Can. Trtmnt. Rev. 5:199-207 [1978]). 2. The ErbB family of receptor tyrosine inases and anti-ErbB antibodies Members of the ErbB family of receptor tyrosine kinases are important mediators of cell growth, differentiation and survival. The receptor family includes four distinct members, including epidermal growth factor receptor (EGFR or ErbBD, HER2 (ErbB2 or p185 m "), HER3 (ErbB3) and HER4 (ErbB4 or tyro2). p185"", was originally identified as the product of the transforming gene from neuroblastomas of chemically treated rats. The activated form of the neu proto-oncogene results from a point mutation (valine to glutamic acid) in the transmembrane region of the encoded protein. Amplification of the human homolog of neu is observed in breast and ovarian cancers and correlates with a poor prognosis (Slamon etal., Science, 235:177-182 (1987); Siamon et al., Science, 244:707-712 (1989); and US Pat No. 4,968,603). To date, no point mutation analogous to that in the /rø/ proto-oncogene has been reported for human tumors. Overexpression of ErbB2 (frequently but not uniformly due to gene amplification) has also been observed in other carcinomas including carcinomas of the stomach, endometrium, salivary gland, lung, kidney, colon, thyroid, pancreas and bladder. See, among others, King et al., Science, 229:974 (1985); Yokota et al., Lancet: 1:765-767 (1986); Fukushigi et al., Mol Cell Biol., 6:955-958 (1986); Geurin et al., Oncogene Res., 3:21-31 (1988); Cohen etal., Oncogene, 4:81-88 (1989); Yonemura etal., Cancer Res., 51:1034 (1991); Borst et al., Gynecol. Oncol., 38:364 (1990); Weiner et al., Cancer Res., 50:421-425 (1990); Kern et al., Cancer Res., 50:5184 (1990); Park et al., Cancer Res., 49:6605 (1989); Zhau et al, Mol. Carcinog., 3:354-357 (1990); Aasland et al. Br. J. Cancer 57:358-363 (1988); Williams et al. Pathobiology 59:46-52 (1991 ); and McCann et al., Cancer, 65:88-92 (1990). ErbB2 may be overexpressed in prostate cancer (Gu et al. Cancer Lett. 99:185-9 (1996); Ross et al. Hum. Pathol. 28:827-33 (1997); Ross et al. Cancer 79:2162-70 (1997); and Sadasivan et al. J. Urol. 150:126-31 (1993)). A spliced form of erbB2 oncogen encoding a constitutively tyrosine phosphorylated ErbB2 receptor is disclosed in PCT publication W0 00/20579, published on April 13, 2000. The erbB2 protein encoded by the splice variant has an in frame deletion of 16 amino acids (CVDLDDKGCPAEQRAS), two of which are conserved cysteine residues. Antibodies directed against the rat p185"" and human ErbB2 protein products have been described. Drebin and colleagues have raised antibodies against the rat neu gene product, p185"". See, for example, Drebin et al.. Cell 41 :695-706 (1985); Myers et al., Meth. Enzym. 198:277-290 (1991 ); and W094/22478. Drebin et al. Oncogene 2:273-277 (1988) report that mixtures of antibodies reactive with two distinct regions of p185" e " result in syπergistic anti-tumor effects on nei transformed NIH-3T3 cells implanted into nude mice. See also U.S. Patent 5,824,311 issued October 20, 1998. Other anti-ErbB2 antibodies with various properties have been described in Tagliabue et al. Int. J. Cancer 47:933-937 (1991); McKenzie eta/. Oncogene 4:543-548 (1989); Mam et al. CancerRes. 51 :5361-5369 (1991); Bacus et al. Molecular Carcinogenesis 3:350-362 (1990); Stancovski etal. PNAS (USA) 88:8691-8695 (1991 ); Bacus et al. Cancer Research 52:2580-2589 (1992); Xu et al. Int. J. Cancer 53:401-408 (1993); W094/00136; Kasprzyk et al. Cancer Research 52:2771-2776 (1992);Hancock et al. CancerRes. 51:4575-4580 (1991); Shawver et al. Cancer Res. 54:1367-1373 (1994); Arteaga et al. Cancer Res. 54:3758-3765 (1994); Harwerth et al. J. Biol. Chem. 267:15160-15167 (1992); U.S. Patent No. 5,783,186; and Klapper et al. Oncogene 14:2099 2109 (1997). Hudziak et al., Mol. Cell. Biol. 9(3): 1165-1172 (1989) describe the generation of a panel of anti-ErbB2 antibodies which were characterized using the human breast tumor cell line SK-BR-3. Relative cell proliferation of the SK-BR-3 cells following exposure to the antibodies was determined by crystal violet staining of the monolayers after 72 hours. Using this assay, maximum inhibition was obtained with the antibody called 4D5 which inhibited cellular proliferation by 56%. Other antibodies in the panel reduced cellular proliferation to a lesser extent in this assay. The antibody 4D5 was further found to sensitize ErbB2-overexpressiπg breast tumor cell lines to the cytotoxic effects of TNF- . See also U.S. Patent No. 5,677,171 issued October 14, 1997. The anti-ErbB2 antibodies discussed in Hudziak et al. are further characterized in Fendly etal. Cancer Research 50:1550-1558 (1990); Kotts et al. In Vitro 26(3):59A (1990); Sarup et al. Growth Regulation 1:72-82 (1991); Shepard et al. J. Clin. Immunol. 1 1(3):117-127 (1991 ); Kumar et al. Mol. Cell. Biol. 11 (2):979-986 (1991 ); Lewis et al. Cancer Immunol. Immunother. 37:255-263 (1993); Pietras et al. Oncogene 9:1829-1838 (1994); Vitetta et al. Cancer Research 54:5301-5309 (1994); Sliwkowski et al. J. Biol. Chem. 269(201:14661-14665 (1994); Scott etal. J. Biol. Chem. 266:14300-5 (1991); D'souza et al. Proc. Natl. Acad. Sci. 91 :7202-7206 (1994); Lewis et al. Cancer Research 56:1457-1465 (1996); and Schaefer et al. Oncogene 15:1385-1394 (1997). The murine monoclonal anti-HER2 antibody inhibits the growth of breast cancer cell lines that overexpress HER2 at the 2+ and 3+ level, but has no activity on cells that express lower levels of HER2 (Lewis et al., Cancer Immunol. Immunother. [1993]). Based on this observation, antibody 4D5 was humanized (Carter et al., Proc. Natl. Acad. Sci. USA 89: 4285-4289 [1992]). The humanized version designated HERCEPTIN ® (huMAb4D5-8, rhuMAb HER2, U.S. Patent No. 5,821 ,337) was tested in breast cancer patients whose tumors overexpress HER2 but who had progressed after conventional chemotherapy (Baselga et al., J. Clin. Oncol. 14:737-744 [1996]); Cobleigh et al, J. Cliπ. Oncol. 17: 2639-2648 [1999]). Most patients in this trial expressed HER2 at the 3+ level, though a fraction was 2+ tumors. Remarkably, HERCEPTIN ® induced clinical responses in 15% of patients (complete responses in 4% of patients, and partial responses in 1 1 %) and the median duration of those responses was 9.1 months. HERCEPTIN ® received marketing approval from the Food and Drug Administration September 25, 1998 for the treatment of patients with metastatic breast cancer whose tumors overexpress the ErbB2 protein. Homology screening has resulted in the identification of two other ErbB receptor family members; ErbB3 (US Pat. Nos. 5,183,884 and 5,480,968 as well as Kraus et al. PNAS (USA) 86:9193-9197 (1989)) and ErbB4 (EP Pat Appln No 599,274; Plowman et al., Proc. Natl. Acad. Sci. USA, 90:1746-1750 (1993); and Plowman et al, Nature, 366:473-475 (1993)). Both of these receptors display increased expression on at least some breast cancer cell lines. 3. Mavtansinoid-antibody conjugates In an attempt to improve their therapeutic index, maytansine and maytansinoids have been conjugated to antibodies specifically binding to tumor cell antigens. Immunoconjugates containing maytansinoids are disclosed, for example, in U.S. Patent Nos. 5,208,020; 5,416,064 and European Patent EP 0 425 235 B1, the disclosures of which are hereby expressly incorporated by reference. Liu et a/., Proc. Natl. Acad. Sci. USA 93:8618-8623 (1996) described immunoconjugates comprising a maytaπsinoid designated DM1 linked to the monoclonal antibody C242 directed against human colorectal cancer. The conjugate was found to be highly cytotoxic towards cultured colon cancer cells, and showed antitumor activity in an in vivo tumor growth assay. Chari et /. Cancer Research 52:127-131 (1992) describe immunoconjugates in which a maytansinoid was conjugated via a disulfide linker to the murine antibody A7 binding to an antigen on human colon cancer cell lines, or to another murine monoclonal antibody TA.1 that binds the HER-2//7e_ oncogene. The cytotoxicity of the TA.1-maytansonoid conjugate was tested in vitro on the human breast cancer cell line SK-BR-3, which expresses 3 x 10 5 HER-2 surface antigens per cell. The drug conjugate achieved a degree of cytotoxicity similar to the free maytansonid drug, which could be increased by increasing the number of maytansinoid molecules per antibody molecule. The A7-maytansinoid conjugate showed low systemic cytotoxicity in mice. Although HERCEPTIN ® is a breakthrough in treating patients with ErbB2-overexpressing breast cancers that have received extensive prior anti-cancer therapy, generally approximately 85% of the patients in this population fail to respond, or respond only poorly, to HERCEPTIN ® treatment, and in the clinical trial preceding marketing approval, the median time to disease progression in all treated patients was only 3.1 months. Therefore, there is a significant clinical need for developing further HER2-directed cancer therapies for those patients with HER2-overexpressing tumors or other diseases associated with HER2 expression that do not respond, or respond poorly, to HERCEPTIN ® treatment. Summary of the Invention The present invention is based on the unexpected experimental finding that HERCEPTIN ® -maytansinoid conjugates are highly effective in the treatment of HER2 (ErbB2) overexpressing tumors that do not respond, or respond poorly, to HERCEPTIN ® therapy. In one aspect, the present invention concerns a method for the treatment of a tumor in a mammal, wherein the tumor is characterized by the overexpression of an ErbB receptor and does not respond or responds poorly to treatment with a monoclonal anti-ErbB antibody, comprising administering to the mammal a therapeutically effective amount of a conjugate of the anti-ErbB antibody with a maytansinoid. In a preferred embodiment, the patient is human. In another preferred embodiment, the ErbB receptor is (human) ErbB2 (HER2). The method is not limited by the mechanism of action of the anti-ErbB antibody used. Thus, the anti-ErbB antiody may, for example, have growth inhibitory properties and/or may induce cell death and/or apoptosis. In a particularly preferred embodiment, the method concerns the treatment of cancer including, without limitation, breast, ovarian, stomach, endometrial, salivary gland, lung, kidney, colon, colorectal, thyroid, pancreatic, prostate and bladder cancer. Preferably the cancer is breast cancer, in particular, breast cancer which overexpresses ErbB2 at a 2+ level or above, more preferably at a 3+ level. A preferred group of antibodies has a biological characteristic of a 4D5 monoclonal antibody, or binds essentially the same epitope as a 4D5 monoclonal antibody, a humanized form of the murine monoclonal antibody 4D5 (ATCC CRL 10463) being particularly preferred. The maytansinoid used in the conjugates of the present invention may be maytansine or, preferably, maytansinol or a maytansinol ester. The antibody and maytansinoid may be conjugated by a bispecific chemical linker, such as N-succinimidyl-4-(2-pyridylthio)propanoate (SPDP) or N-succinimidyl-4-(2-pyridylthio)pentanoate (SPP). The linking group between the antibody and the maytansinoid may, for example, be a disulfide, thioether, acid labile, photolabile, peptidase labile, or esterase labile group. In another aspect, the invention concerns an article of manufacture comprising a container and a composition contained therein, wherein the composition comprises an anti-ErbB antibody-maytansiπoid conjugate, and further comprising a package insert or label indicating that the composition can be used to treat cancer characterized by overexpression of an ErbB receptor, preferably at a 2+ level or above. Brief Desctiption of the Drawings Figure 1 shows the structure of the maytansinoid, designated "DM1." Figure 2 illustrates the structure of a HERCEPTIN ® -DM1 conjugate. Figure 3 is the elution profile of HERCEPTIN ® -DM1 conjugate on a Sephacryl S300 gel filtration column. Figure 4 shows the nucleotide sequence of a HER2 transgene plasmid construct (SEQ ID NO: 1) directing the expression of native human HER2 (ErbB2) in the mammary gland of a transgenic mouse. The figure includes the nucleotide sequence of HER2 (ErbB2) cDNA insert (SEQ ID NO: 2) as well as the deduced amino acid sequence of HER2 (ErbB2) (SEQ ID NO: 3), including the signal sequence. Within SEQ ID NO: 3, residues from about 22 to about 645, inclusive represent the HER2 (ErbB2) extracellular domain. Figure 5 illustrates the effect of HERCEPTIN ® -DM1 on HER2-transgenic tumors. Two mm 3 pieces of MMTV- HER2-transgenic tumors were transplanted into the mammary fat pad of FVB mice. When tumors reached 250 mm 3 , groups of 8 mice were injected i.v. on 5 consecutive days with a HERCEPTIN ® -DM1 conjugate. Two other groups of mice were treated IP twice per week with 10 mg/kg of either HERCEPTIN ® or RITUXAN ® . Figure 6 shows the heavy chain variable region sequence of a humanized anti-HER2 antibody 2C4. Figure 7 shows the light chain variable region sequence of a humanized anti-HER2 antibody 2C4. Detailed Description of the Invention 1. Definitions Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Singleton et al., Dictionary of Microbiology and Molecular Biology 2nd ed., J. Wiley & Sons (New York, NY 1994). One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present invention. Indeed, the present invention is in no way limited to the methods and materials described. For purposes of the present invention, the following terms are defined below. An "ErbB receptor" or "ErbB" is a receptor protein tyrosine kinase which belongs to the ErbB receptor family and includes ErbB1 (EGFR), ErbB2 (HER2), ErbB3 (HER3) and ErbB4 (HER4) receptors and other members of this family to be identified in the future. The definition specifically includes ErbB receptors encoded by spliced forms of the corresponding erbB oncogens, including, without limitation, the deletion variant of ErbB2 disclosed in PCT publication No. WO 00/20579 (published on April 13, 2000). The ErbB receptor will generally comprise an extracellular domain, which may bind an ErbB ligand; a lipophilic transmembrane domain; a conserved intracellular tyrosine kinase domain; and a carboxyl-terminal signaling domain harboring several tyrosine residues which can be phosphorylated. The ErbB receptor may be a "native sequence" ErbB receptor or a functional derivative, such as an "amino acid sequence variant" thereof. Preferably the ErbB receptor is native sequence human ErbB receptor. The terms "ErbBI ", "epidermal growth factor receptor" and "EGFR" are used interchangeably herein and refer to native sequence EGFR as disclosed, for example, in Carpenter et al. Ann. Rev. Biochem. 56:881 -914 (1987), including naturally occurring mutant forms thereof (e.g. a deletion mutant EGFR as in Humphrey etal. PNAS (USA) 87:4207-4211 (1990)), and its functional derivatives, such as amino acid sequence variants. erbB] refers to the gene encoding the EGFR protein product. The expressions "ErbB2" and "HER2" are used interchangeably herein and refer to native sequence human HER2 protein described, for example, in Semba et al., PNAS (USA) 82:6497-6501 (1985) and Yamamoto et al. Nature 319:230-234 (1986) (Genebank accession number X03363), and functional derivatives, such as amino acid sequnce variants thereof. The term erbB2 refers to the gene encoding human HER2 and neu refers to the gene encoding rat p185" e ". Preferred HER2 is native sequence human HER2. Examples of antibodies which bind HER2 include MAbs 4D5 (ATCC CRL 10463), 2C4 (ATCC HB-12697), 7F3 (ATCC HB-12216), and 7C2 (ATCC HB 12215) (see, US Patent No. 5,772,997; W098/77797; and US Patent No. 5,840,525, expressly incorporated herein by reference). Humanized anti-HER2 antibodies include huMAb4D5-1, huMAb4D5-2, huMAb4D5-3, huMAb4D5-4, huMAb4D5-5, huMAb4D5-6, huMAb4D5-7 and huMAb4D5-8 (HERCEPTIN®) as described in Table 3 of U.S. Patent 5,821 ,337 expressly incorporated herein by reference; humanized 520C9 (W093/21319). Human anti HER2 antibodies are described in U.S. Patent No. 5,772,997 issued June 30, 1998 and WO 97/00271 published January 3, 1997. "ErbB3" and "HER3" refer to the receptor polypeptide as disclosed, for example, in US Pat. Nos. 5,183,884 and 5,480,968 as well as Kraus et al. PNAS (USA) 86:9193-9197 (1989), and functional derivatives, including amino acid sequence variants thereof. Examples of antibodies which bind HER3 are described in US Patent No. 5,968,511 (Akita and Sliwkowski), e.g. the 8B8 antibody (ATCC HB 12070) or a humanized variant thereof. The terms "ErbB4" and "HER4" herein refer to the receptor polypeptide as disclosed, for example, in EP Pat Appln No 599,274; Plowman et al, Proc. Natl. Acad. Sci. USA, 90:1746-1750 (1993); and Plowman et al, Nature, 366:473-475 (1993), and functional derivatives, including amino acid sequence variants thereof such as the HER4 isoforms disclosed in WO 99/19488. A "native" or "native sequence" EGFR, HER2, HER3 or HER4 polypeptide may be isolated from nature, produced by techniques of recombinant DNA technology, chemically synthesized, or produced by any combinations of these or similar methods. "Functional derivatives" include amino acid sequence variants, and covalent derivatives of the native polypeptides as long as they retain a qualitative biological activity of the corresponding native polypeptide. Amino acid sequence variants generally differ from a native sequence in the substitution, deletion and/or insertion of one or more amino acids anywhere within a native amino acid sequence. Deletional variants include fragments of the native polypeptides, and variants having N- and/or C-terminal truncations. Ordinarily, amino acid sequence variants will possess at least about 70% homology, preferably at least about 80%, more preferably at least about 90% homology with a native polypeptide. "Homology" is defined as the percentage of residues in the amino acid sequence variant that are identical after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent homology. Methods and computer programs for the alignment are well known in the art. One such computer program is "Align 2", authored by Geπentech, Inc., which was filed with user documentation in the United States Copyright Office, Washington, DC 20559, on December 10, 1991. By "ErbB ligand" is meant a polypeptide which binds to and/or activates an ErbB receptor. The ErbB ligand of particular interest herein is a native sequence human ErbB ligand such as Epidermal Growth Factor (EGF) (Savage et al, J. Biol. Chem. 247:7612-7621 (1972)); Taπsforming Growth Factor alpha (TGF-alpha) (Marquardt et a . Science 223:1079-1082 (1984)); amphiregulin also known as schwanoma or keratiπocyte autocrine growth factor (Shoyab et al. Science 243:1074-1076 (1989); Ki ura et al. Nature 348:257-260 (1990); and Cook et al. Mol. Cell. Biol. 11 :2547-2557 (1991)); betacellulin (Shing et al, Science 259:1604-1607 (1993); and Sasada et al. Biochem. Biophys. Res. Commun. 190:1173 (1993)); heparin-binding epidermal growth factor (HB-EGF) (Higashiyama et al, Science 251 :936-939 (1991)); epiregulin (Toyoda etal, J. Biol. Chem. 270:7495-7500 (1995); and Komurasaki et al. Oncogene 15:2841 -2848 (1997)), a heregulin (see below); neuregulin-2 (NRG-2) (Carraway et al, Nature 387:512- 516 (1997)); neuregulin-3 (NRG-3) (Zhang et al., Proc. Natl. Acad. Sci. 94:9562-9567 (1997)); or cripto (CR-1) (Kannan et al. J. Biol. Chem. 272(6):3330-3335 (1997)). ErbB ligands which bind EGFR include EGF, TGF-alpha, amphiregulin, betacellulin, HB-EGF and epiregulin. ErbB ligands which bind HER3 include heregulins. ErbB ligands capable of binding HER4 include betacellulin, epiregulin, HB-EGF, NRG-2, NRG-3 and heregulins. "Heregulin" (HRG) when used herein refers to a polypeptide which activates the ErbB2-ErbB3 and ErbB2- ErbB4 protein complexes (i.e. induces phosphorylation of tyrosine residues in the complex upon binding thereto). Various heregulin polypeptides encompassed by this term are disclosed in Holmes et al. Science 256:1205-1210 (1992); WO 92/20798; Wen et al, Mol. Cell. Biol. 14(3):1909-1919 (1994) and Marchionni et al. Nature 362:312- 318 (1993), for example. The term includes biologically active fragments and/or variants of a naturally occurring HRG polypeptide, such as an EGF-like domain fragment thereof (e.g. HRGβ,,^). An "ErbB hetero-oligomer" herein is a noncovalently associated oligomer comprising at least two different ErbB receptors. Such complexes may form when a cell expressing two or more ErbB receptors is exposed to an ErbB ligand and can be isolated by immunoprecipitation and analyzed by SDS-PAGE as described in Sliwkowski et al, J. Bio Chem., 269(20):14661 -14665 (1994), for example. Examples of such ErbB hetero-oligomers include EGFR- HER2, HER2-HER3 and HER3-HER4 complexes. Moreover, the ErbB hetero-oligomer may comprise two or more HER2 receptors combined with a different ErbB receptor, such as HER3, HER4 or EGFR. Other proteins, such as a cytokine receptor subunit [e.g. gp130), may be included in the hetero-oligomer. In the context of HER2 variants, such as HER2 fragments, the phrase "having the biological activity of a native human HER2" is used to refer to the qualitative ability of such fragments to induce tumor growth when overexpressed in an animal model (transgenic or non-transgenic) of the present invention. "Tumor", as used herein, refers to all neoplastic cell growth and proliferation, whether malignant or benign, and all pre-caπcerous and cancerous cells and tissues. The terms "cancer" and "cancerous" refer to or describe the physiological condition in mammals that is typically characterized by unregulated cell growth. Examples of cancer include, but are not limited to, carcinoma, lymphoma, blastoma, sarcoma, and leukemia or lymphoid malignancies. More particular examples of such cancers include squamous cell cancer {e.g. epithelial squamous cell cancer), lung cancer including small-cell lung cancer, non- small cell lung cancer, adenocarcinoma of the lung and squamous carcinoma of the lung, cancer of the peritoneum, hepatocellular cancer, gastric or stomach cancer including gastrointestinal cancer, pancreatic cancer, glioblastoma, cervical cancer, ovarian cancer, liver cancer, bladder cancer, hepatoma, breast cancer, colon cancer, rectal cancer, colorectal cancer, endometrial or uterine carcinoma, salivary gland carcinoma, kidney or renal cancer, prostate cancer, vulval cancer, thyroid cancer, hepatic carcinoma, anal carcinoma, penile carcinoma, as well as head and neck cancer. A cancer which "overexpresses" an ErbB receptor is one which has significantly higher levels of an ErbB receptor, such as HER2, at the cell surface thereof, compared to a πoncancerous cell of the same tissue type. Such overexpression may be caused by gene amplification or by increased transcription or translation. ErbB receptor overexpression may be determined in a diagnostic or prognostic assay by evaluating increased levels of the ErbB protein present on the surface of a cell (e.g. via an immuπohistochemistry assay; IHC). Alternatively, or additionally, one may measure levels of ErbB-encoding nucleic acid in the cell, e.g. via fluorescent in situ hybridization (FISH; see W098/45479 published October, 1998), Southern blotting, or polymerase chain reaction (PCR) techniques, such as real time quantitative PCR (RT-PCR). One may also study ErbB receptor overexpression by measuring shed antigen (e.g., ErbB extracellular domain) in a biological fluid such as serum (see, e.g., U.S. Patent No. 4,933,294 issued June 12, 1990; W091/05264 published April 18, 1991; U.S. Patent 5,401 ,638 issued March 28, 1995; and Sias et al. J. Immunol. Methods 132: 73-80 (1990)). Aside from the above assays, various in vivo assays are available to the skilled practitioner. For example, one may expose cells within the body of the patient to an antibody which is optionally labeled with a detectable label, e.g. a radioactive isotope, and binding of the antibody to cells in the patient can be evaluated, e.g. by external scanning for radioactivity or by analyzing a biopsy taken from a patient previously exposed to the antibody. The tumors overexpressing HER2 are rated by immunohistochemical scores corresponding to the number of copies of HER2 molecules expressed per cell, and can been determined biochemically: 0 « 0-10,000 copies/cell, 1 + = at least about 200,000 copies/cell, 2+ = at least about 500,000 copies/cell, 3+ = at least about 2,000,000 copies/cell. Overexpression of HER2 at the 3+ level, which leads to ligand-independent activation of the tyrosine kinase (Hudziak et al, Proc. Natl Acad. Sci. USA 84: 7159-7163 [1987]), occurs in approximately 30% of breast cancers, and in these patients, relapse-free survival and overall survival are diminished (Slamon etal, Science 244: 707-712 [1989]; Slamon et al. Science 235: 177-182 [1987]). Conversely, a cancer which is "not characterized by overexpression of an ErbB receptor" is one which, in a diagnostic assay, does not express higher than normal levels of ErbB receptor compared to a noncancerous cell of the same tissue type. A "hormone independent" cancer is one in which proliferation thereof is not dependent on the presence of a hormone which binds to a receptor expressed by cells in the cancer. Such cancers do not undergo clinical regression upon administration of pharmacological or surgical strategies that reduce the hormone concentration in or near the tumor. Examples of hormone independent cancers include androgen independent prostate cancer, estrogen independent breast cancer, endometrial cancer and ovarian cancer. Such cancers may begin as hormone dependent tumors and progress from a hormone-sensitive stage to a hormone-refractory tumor following anti-hormonal therapy. The term "antibody" herein is used in the broadest sense and specifically covers intact monoclonal antibodies, polyclonal antibodies, multispecific antibodies (e.g. bispecific antibodies) formed from at least two intact antibodies, and antibody fragments, so long as they exhibit the desired biological activity. The term "monoclonal antibody" as used herein refers to an antibody obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally occurring mutations that may be present in minor amounts. Monoclonal antibodies are highly specific, being directed against a single antigenic site. Furthermore, in contrast to polyclonal antibody preparations which include different antibodies directed against different determinants (epitopes), each monoclonal antibody is directed against a single determinant on the antigen. In addition to their specificity, the monoclonal antibodies are advantageous in that they may be synthesized uncontaminated by other antibodies. The modifier "monoclonal" indicates the character of the antibody as being obtained from a substantially homogeneous population of antibodies, and is not to be construed as requiring production of the antibody by any particular method. For example, the monoclonal antibodies to be used in accordance with the present invention may be made by the hybridoma method first described by Kohler et al., Nature, 256:495 (1975), or may be made by recombinant DNA methods (see, e.g., U.S. Patent No. 4,816,567). The "monoclonal antibodies" may also be isolated from phage antibody libraries using the techniques described in Clackson et al, Nature, 352:624-628 (1991 ) and Marks et al, J. Mol. Biol, 222:581-597 (1991), for example. The monoclonal antibodies herein specifically include "chimeric" antibodies in which a portion of the heavy and/or light chain is identical with or homologous to corresponding sequences in antibodies derived from a particular species or belonging to a particular antibody class or subclass, while the remainder of the chain(s) is identical with or homologous to corresponding sequences in antibodies derived from another species or belonging to another antibody class or subclass, as well as fragments of such antibodies, so long as they exhibit the desired biological activity (U.S. Patent No. 4,816,567; and Morrison et al, Proc. Natl. Acad. Sci. USA, 81:6851-6855 (1984)). Chimeric antibodies of interest herein include primatized antibodies comprising variable domain antigen-binding sequences derived from a non- human primate (e.g. Old World Monkey, Ape etc) and human constant region sequences. "Antibody fragments" comprise a portion of an intact antibody, preferably comprising the antigen-binding or variable region thereof. Examples of antibody fragments include Fab, Fab', F(ab') 2 , and Fv fragments; diabodies; linear antibodies; single-chain antibody molecules; and multispecific antibodies formed from antibody fragment(s). An "intact" antibody is one which comprises an antigen-binding variable region as well as a light chain constant domain (C L ) and heavy chain constant domains, C H 1, C H 2 and C H 3. The constant domains may be native sequence constant domains (e.g. human native sequence constant domains) or amino acid sequence variant thereof. Preferably, the intact antibody has one or more effector functions. "Humanized" forms of non-human (e.g., rodent) antibodies are chimeric antibodies that contain minimal sequence derived from non-human immunoglobulin. For the most part, humanized antibodies are human immunoglobuliπs (recipient antibody) in which residues from a hypervariable region of the recipient are replaced by residues from a hypervariable region of a non-human species (donor antibody) such as mouse, rat, rabbit or nonhuman primate having the desired specificity, affinity, and capacity. In some instances, framework region (FR) residues of the human immunoglobuiin are replaced by corresponding non-human residues. Furthermore, humanized antibodies may comprise residues that are not found in the recipient antibody or in the donor antibody. These modifications are made to further refine antibody performance. In general, the humanized antibody will comprise substantially all of at least one, and typically two, variable domains, in which all or substantially all of the hypervariable loops correspond to those of a non-human immunoglobuiin and all or substantially all of the FRs are those of a human immunoglobuiin sequence. The humanized antibody optionally also will comprise at least a portion of an immunoglobuiin constant region (Fc), typically that of a human immunoglobuiin. For further details, see Jones et al, Nature 321 :522-525 (1986); Riechmann et al, Nature 332:323-329 (1988); and Presta, Curr. Op. Struct. Biol. 2:593-596 (1992). Humanized anti-ErbB2 antibodies include huMAb4D5-1 , huMAb4D5-2, huMAb4D5-3, huMAb4D5-4, huMAb4D5-5, huMAb4D5-6, huMAb4D5-7 and huMAb4D5-8 (HERCEPTIN*) as described in Table 3 of U.S. Patent 5,821 ,337 expressly incorporated herein by reference; humanized 520C9 (W093/21319) and humanized 2C4 antibodies as those shown in Figure 6. Antibody "effector functions" refer to those biological activities attributable to the Fc region (a native sequence Fc region or amino acid sequence variant Fc region) of an antibody. Examples of antibody effector functions include C1 q binding; complement dependent cytotoxicity; Fc receptor binding; antibody-dependent cell-mediated cytotoxicity (ADCC); phagocytosis; down regulation of cell surface receptors (e.g. B cell receptor; BCR), etc. Depending on the amino acid sequence of the constant domain of their heavy chains, intact antibodies can be assigned to different "classes". There are five major classes of intact antibodies: IgA, IgD, IgE, IgG, and IgM, and several of these may be further divided into "subclasses" (isotypes), e.g., IgGI , lgG2, lgG3, lgG4, IgA, and lgA2. The heavy-chain constant domains that correspond to the different classes of antibodies are called , , , , and , respectively. The subuπit structures and three-dimensional configurations of different classes of immunoglobulins are well known. "Antibody-dependent cell-mediated cytotoxicity" and "ADCC" refer to a cell-mediated reaction in which nonspecific cytotoxic cells that express Fc receptors (FcRs) (e.g. Natural Killer (NK) cells, neutrophils, and macrophages) recognize bound antibody on a target cell and subsequently cause lysis of the target cell. The primary cells for mediating ADCC, NK cells, express Fc Rill only, whereas monocytes express Fc Rl, Fc Rll and Fc Rill. FcR expression on hematopoietic cells in summarized is Table 3 on page 464 of Ravetch and Kinet, Aπnu. Rev. Immunol 9:457-92 (1991). To assess ADCC activity of a molecule of interest, an in vitro ADCC assay, such as that described in US Patent No. 5,500,362 or 5,821,337 may be performed. Useful effector cells for such assays include peripheral blood mononuclear cells (PBMC) and Natural Killer (NK) cells. Alternatively, or additionally, ADCC activity of the molecule of interest may be assessed in vivo, e.g., in a animal model such as that disclosed in Clγnes et al. PNAS (USA) 95:652-656 (1998). "Human effector cells" are leukocytes which express one or more FcRs and perform effector functions. Preferably, the cells express at least Fc Rill and perform ADCC effector function. Examples of human leukocytes which mediate ADCC include peripheral blood mononuclear cells (PBMC), natural killer (NK) cells, monocytes, cytotoxic T cells and neutrophils; with PBMCs and NK cells being preferred. The effector cells may be isolated from a native source thereof, e.g. from blood or PBMCs as described herein. The terms "Fc receptor" or "FcR" are used to describe a receptor that binds to the Fc region of an antibody. The preferred FcR is a native sequence human FcR. Moreover, a preferred FcR is one which binds an IgG antibody (a gamma receptor) and includes receptors of the Fc Rl, Fc Rll, and Fc Rill subclasses, including allelic variants and alternatively spliced forms of these receptors. Fc Rll receptors include Fc RIIA (an "activating receptor") and Fc RUB (an "inhibiting receptor"), which have similar amino acid sequences that differ primarily in the cytoplasmic domains thereof. Activating receptor Fc RIIA contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain. Inhibiting receptor Fc RUB contains an immunoreceptor tyrosine-based inhibition motif (ITIM) in its cytoplasmic domain, (see review M. in Daeron, Annu. Rev. Immunol. 15:203-234 (1997)). FcRs are reviewed in Ravetch and Kinet, Annu. Rev. Immunol 9:457-92 (1991 ); Capel et al, Immunomethods 4:25-34 (1994); and de Haas et al, J. Lab. Clin. Med. 126:330-41 (1995). Other FcRs, including those to be identified in the future, are encompassed by the term "FcR" herein. The term also includes the neonatal receptor, FcRn, which is responsible for the transfer of maternal IgGs to the fetus (Guyer et al, J. Immunol. 1 17:587 (1976) and Kim et al, J. Immunol. 24:249 (1994)). "Complement dependent cytotoxicity" or "CDC" refers to the ability of a molecule to lyse a target in the presence of complement. The complement activation pathway is initiated by the binding of the first component of the complement system (C1q) to a molecule (e.g. an antibody) complexed with a cognate antigen. To assess complement activation, a CDC assay, e.g. as described in Gazzano-Santoro et al, J. Immunol. Methods 202:163 (1996), may be performed. "Native antibodies" are usually heterotetrameric glycoproteins of about 150,000 daltons, composed of two identical light (L) chains and two identical heavy (H) chains. Each light chain is linked to a heavy chain by one covalent disulfide bond, while the number of disulfide linkages varies among the heavy chains of different immunoglobuiin isotypes. Each heavy and light chain also has regularly spaced intrachain disulfide bridges. Each heavy chain has at one end a variable domain (V H ) followed by a number of constant domains. Each light chain has a variable domain at one end (V L ) and a constant domain at its other end. The constant domain of the light chain is aligned with the first constant domain of the heavy chain, and the light-chain variable domain is aligned with the variable domain of the heavy chain. Particular amino acid residues are believed to form an interface between the light chain and heavy chain variable domains. The term "variable", as used in connection with antibodies, refers to the fact that certain portions of the antibody variable domains differ extensively in sequence among antibodies and are used in the binding and specificity of each particular antibody for its particular antigen. However, the variability is not evenly distributed throughout the variable domains of antibodies. It is concentrated in three segments called hypervariable regions both in the light chain and the heavy chain variable domains. The more highly conserved portions of variable domains are called the framework regions (FRs). The variable domains of native heavy and light chains each comprise four FRs, largely adopting a -sheet configuration, connected by three hypervariable regions, which form loops connecting, and in some cases forming part of, the -sheet structure. The hypervariable regions in each chain are held together in close proximity by the FRs and, with the hypervariable regions from the other chain, contribute to the formation of the antigen-binding site of antibodies (see Kabat etal, Sequences of Proteins of Immunological Interest, 5th Ed. Public Health Service, National Institutes of Health, Bethesda, MD. (1991)). The constant domains are not involved directly in binding an antibody to an antigen, but exhibit various effector functions, such as participation of the antibody in antibody dependent cellular cytotoxicity (ADCC). The term "hypervariable region" when used herein refers to the amino acid residues of an antibody which are responsible for antigen-binding. The hypervariable region generally comprises amino acid residues from a "complementarity determining region" or "CDR" (e.g. residues 24-34 (L1), 50-56 (L2) and 89-97 (L3) in the light chain variable domain and 31-35 (H1), 50-65 (H2) and 95-102 (H3) in the heavy chain variable domain; Kabat et al, Sequences of Proteins of Immunological Interest, 5th Ed. Public Health Service, National Institutes of Health, Bethesda, MD. (1991 )) and/or those residues from a "hypervariable loop" (e.g. residues 26-32 (L1 ), 50-52 (L2) and 91 - 96 (L3) in the light chain variable domain and 26-32 (H1 ), 53-55 (H2) and 96-101 (H3) in the heavy chain variable domain; Chothia and Lesk J. Mol. Biol. 196:901-917 (1987)). "Framework Region" or "FR" residues are those variable domain residues other than the hypervariable region residues as herein defined. An "isolated" antibody is one which has been identified and separated and/or recovered from a component of its natural environment. Contaminant components of its natural environment are materials which would interfere with diagnostic or therapeutic uses for the antibody, and may include enzymes, hormones, and other proteiπaceous or noπproteinaceous solutes. In preferred embodiments, the antibody will be purified (1 ) to greater than 95% by weight of antibody as determined by the Lowry method, and most preferably more than 99% by weight, (2) to a degree sufficient to obtain at least 15 residues of N-terminal or internal amino acid sequence by use of a spinning cup sequenator, or (3) to homogeneity by SDS-PAGE under reducing or nonreducing conditions using Coomassie blue or, preferably, silver stain. Isolated antibody includes the antibody in situ within recombinant cells since at least one component of the antibody's natural environment will not be present. Ordinarily, however, isolated antibody will be prepared by at least one purification step. An antibody "which binds" an antigen of interest, e.g. ErbB2 antigen, is one capable of binding that antigen with sufficient affinity such that the antibody is useful as a diagnostic and/or therapeutic agent in targeting a cell expressing the antigen and/or for targeted delivery of a cytotoxic or other chemotherapeutic agent, such as a maytansinoid. Where the antibody is one which binds ErbB2, it will usually preferentially bind ErbB2 as opposed to other ErbB receptors, and may be one which does not significantly cross-react with other proteins such as EGFR, ErbB3 or ErbB4. In such embodiments, the extent of binding of the antibody to these πon-ErbB2 proteins (e.g., cell surface binding to endogenous receptor) will be less than 10% as determined by fluorescence activated cell sorting (FACS) analysis or radioimmunoprecipitation (RIA). Sometimes, the anti-ErbB2 antibody will not significantly cross- react with the rat neu protein, e.g., as described in Schecter et al. Nature 312:513 (1984) and Drebin et al, Nature 312:545-548 (1984). Unless indicated otherwise, the expressions "monoclonal antibody 4D5", and "4D5 monoclonal antibody" refer to an antibody that has antigen binding residues of, or derived from, the murine 4D5 antibody. For example, the monoclonal antibody 4D5 may be murine monoclonal antibody 4D5 (ATCC CRL 10463) or a variant thereof, such as humanized antibody 4D5, possessing antigen binding amino acid residues of murine monoclonal antibody 4D5. Exemplary humanized 4D5 antibodies include huMAb4D5-1, huMAb4D5-2, huMAb4D5-3, huMAb4D5-4, huMAb4D5-5, huMAb4D5-6, huMAb4D5-7 and huMAb4D5-8 (HERCEPTIN®) as in US Patent No. 5,821 ,337, with huMAb4D5-8 (HERCEPTIN®) being a preferred humanized 4D5 antibody. An antibody having a "biological characteristic" of a designated antibody, such as the monoclonal antibody designated 4D5, is one which possesses one or more of the biological characteristics of that antibody which distinguish it from other antibodies that bind to the same antigen (e.g. ErbB2). For example, an antibody with a biological characteristic of 4D5 may show growth inhibitory effect on ErbB2 overexpressing cells in a manner that is dependent on the ErbB2 expression level and/or bind the same epitope in the extracellular domain of ErbB2 as that bound by 4D5 (e.g. which blocks binding of monoclonal antibody 4D5 to ErbB2). A "growth inhibitory agent" when used herein refers to a compound or composition which inhibits growth of a cell, especially an ErbB expressing cancer cell either //? vitro or in vivo. Thus, the growth inhibitory agent may be one which significantly reduces the percentage of ErbB expressing cells in S phase. Examples of growth inhibitory agents include agents that block cell cycle progression (at a place other than S phase), such as agents that induce G1 arrest and M-phase arrest. Classical M-phase blockers include the vincas (vincristine and vinblastine), taxanes, and topo II inhibitors such as doxorubicin, epirubicin, daunorubicin, etoposide, and bleomycin. Those agents that arrest G1 also spill over into S-phase arrest, for example, DNA alkylating agents such as tamoxifen, prednisone, dacarbazine, mechlorethamine, cisplatin, methotrexate, 5-fluorouracil, and ara-C. Further information can be found in The Molecular Basis of Cancer, Mendelsohn and Israel, eds., Chapter 1, entitled "Cell cycle regulation, oncogenes, and antineoplastic drugs" by Murakami et al. (WB Saunders: Philadelphia, 1995), especially p. 13. Examples of "growth inhibitory" antibodies are those which bind to ErbB2 and inhibit the growth of cancer cells overexpressing ErbB2. Preferred growth inhibitory anti-ErbB2 antibodies inhibit growth of SK-BR-3 breast tumor cells in cell culture by greater than 20%, and preferably greater than 50% (e.g. from about 50% to about 100%) at an antibody concentration of about 0.5 to 30 g/ml, where the growth inhibition is determined six days after exposure of the SK-BR-3 cells to the antibody (see U.S. Patent No. 5,677,171 issued October 14, 1997). The SK-BR-3 cell growth inhibition assay is described in more detail in that patent and hereinbelow. The preferred growth inhibitory antibody is monoclonal antibody 4D5, e.g., humanized 4D5. A molecule (e.g. antibody) which "induces cell death" is one which causes a viable cell to become nonviable. The cell is generally one which expresses the ErbB2 receptor, especially where the cell overexpresses the ErbB2 receptor. Preferably, the cell is a cancer cell, e.g. a breast, ovarian, stomach, endometrial, salivary gland, lung, kidney, colon, thyroid, pancreatic, prostate or bladder cancer cell. In vitro, the cell may be a SK-BR-3, BT474, Calu 3, MDA- MB-453, MDA-MB-361 or SK0V3 cell. Cell death in vitro may be determined in the absence of complement and immune effector cells to distinguish cell death induced by antibody-dependent cell-mediated cytotoxicity (ADCC) or complement dependent cytotoxicity (CDC). Thus, the assay for cell death may be performed using heat inactivated serum (i.e. in the absence of complement) and in the absence of immune effector cells. To determine whether the molecule is able to induce cell death, loss of membrane integrity as evaluated by uptake of propidium iodide (PI), trypan blue (see Moore et al. Cytotechnology • 17:1 -1 1 (1995)) or 7AAD can be assessed relative to untreated cells. Preferred cell death-inducing antibodies are those which induce PI uptake in the PI uptake assay in BT474 cells. Examples of antibodies which induce cell death include anti-ErbB2 antibodies 7C2 and 7F3 (WO 98/17797, expressly incorporated herein by reference), including humanized and/or affinity matured variants thereof. A molecule (e.g. antibody) which "induces apoptosis" is one which induces programmed cell death as determined by binding of anπexin V, fragmentation of DNA, cell shrinkage, dilation of endoplasmic reticulum, cell fragmentation, and/or formation of membrane vesicles (called apoptotic bodies). The cell is usually one which overexpresses the ErbB2 receptor. Preferably the cell is a tumor cell, e.g. a breast, ovarian, stomach, endometrial, salivary gland, lung, kidney, colon, thyroid, pancreatic, prostate or bladder cancer cell. In vitro, the cell may be a SK- BR-3, BT474, Calu 3 cell, MDA-MB-453, MDA-MB-361 or SK0V3 cell. Various methods are available for evaluating the cellular events associated with apoptosis. For example, phosphatidyl serine (PS) translocation can be measured by annexin binding; DNA fragmentation can be evaluated through DNA laddering; and nuclear/chromatin condensation along with DNA fragmentation can be evaluated by any increase in hypodiploid cells. Preferably, the molecule which induces apoptosis is one which results in about 2 to 50 fold, preferably about 5 to 50 fold, and most preferably about 10 to 50 fold, induction of annexin binding relative to untreated cell in an annexin binding assay using BT474 cells. Sometimes the pro-apoptotic molecule will be one which further blocks ErbB ligand activation of an ErbB receptor. In other situations, the molecule is one which does not significantly block ErbB ligand activation of an ErbB receptor. Further, the molecule may induce apoptosis, without inducing a large reduction in the percent of cells in S phase (e.g. one which only induces about 0-10% reduction in the percent of these cells relative to control). Examples of antibodies which induce apoptosis include anti-ErbB2 antibodies 7C2 and 7F3 (WO 98/17797, expressly incorporated herein by reference), including humanized and/or affinity matured variants thereof. An antibody which "blocks" ligand activation of an ErbB receptor is one which reduces or prevents such activation as hereinabove defined, wherein the antibody is able to block ligand activation of the ErbB receptor substantially more effectively than monoclonal antibody 4D5, e.g. about as effectively as monoclonal antibodies 7F3 or 2C4 or Fab fragments thereof and preferably about as effectively as monoclonal antibody 2C4 or a Fab fragment thereof. For example, the antibody that blocks ligand activation of an ErbB receptor may be one which is about 50- 100% more effective than 4D5 at blocking formation of an ErbB hetero-oligomer. Blocking of ligand activation of an ErbB receptor can occur by any means, e.g. by interfering with: ligand binding to an ErbB receptor, ErbB complex formation, tyrosine kinase activity of an ErbB receptor in an ErbB complex and/or phosphorylation of tyrosine kinase residue(s) in or by an ErbB receptor. Examples of antibodies which block ligand activation of an ErbB receptor include monoclonal antibodies 2C4 and 7F3 (which block HRG activation of ErbB2/ErbB3 and ErbB2/ErbB4 hetero-oligomers; and EGF, TGF- , amphiregulin, HB-EGF and/or epiregulin activation of an EGFR/ErbB2 hetero-oligomer); and L26, L96 and L288 antibodies (Klapper et al. Oncogene 14:2099-2109 (1997)), which block EGF and NDF binding to T47D cells which express EGFR, ErbB2, ErbB3 and ErbB4. Humanized and/or affinity matured variants these and other antibodies within the definition are specifically included. The term "epitope" is used to refer to binding sites for (monoclonal or polyclonal) antibodies on protein antigens. Antibodies that bind to a certain epitope are identified by "epitope mapping." There are many methods known in the art for mapping and characterizing the location of epitopes on proteins, including solving the crystal structure of an antibody-antigen complex, competition assays, gene fragment expression assays, and synthetic peptide-based assays, as described, for example, in Chapter 1 1 of Harlow and Lane, Using Antibodies, a Laboratory Manual, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, 1999. Competition assays are discussed below. According to the gene fragment expression assays, the open reading frame encoding the protein is fragmented either randomly or by specific genetic constructions and the reactivity of the expressed fragments of the protein with the antibody to be tested is determined. The gene fragments may, for example, be produced by PCR and then transcribed and translated into protein in vitro, in the presence of radioactive amino acids. The binding of the antibody to the radioactively labeled protein fragments is then determined by immunoprecipitation and gel electrophoresis. Certain epitopes can also be identified by using large libraries of random peptide sequences displayed on the surface of phage particles (phage libraries). Alternatively, a defined library of overlapping peptide fragments can be tested for binding to the test antibody in simple binding assays. The latter approach is suitable to define linear epitopes of about 5 to 15 amino acids. An antibody binds "essentially the same epitope" as a reference antibody, when the two antibodies recognize identical or sterically overlapping epitopes. The most widely used and rapid methods for determining whether two epitopes bind to identical or sterically overlapping epitopes are competition assays, which can be configured in all number of different formats, using either labeled antigen or labeled antibody. Usually, the antigen is immobilized on a 96-well plate, and the ability of unlabeled antibodies to block the binding of labeled antibodies is measured using radioactive or enzyme labels. The "epitope 4D5" is the region in the extracellular domain of ErbB2 to which the antibody 4D5 (ATCC CRL 10463) binds. This epitope is close to the transmembrane domain of ErbB2, and extends from about residue 519 to about residue 625, inclusive within the ErbB2 extracellular domain sequence included in SEQ ID NO: 3, Figure 4. To screen for antibodies which bind to the 4D5 epitope, a routine cross-blocking assay such as that described in Harlow and Lane, supra, can be performed. Alternatively, epitope mapping can be performed to assess whether the antibody binds to the 4D5 epitope of ErbB2 (e.g. any one or more residues in the region from about residue 529 to about residue 625, inclusive in SEQ ID NO: 3). The "epitope 3H4" is the region in the extracellular domain of ErbB2 to which the antibody 3H4 binds. This epitope includes residues from about 541 to about 599, inclusive, in the amino acid sequence of ErbB2 extracellular domain (see Figure 4 and SEQ ID NO: 3). The "epitope 7C2/7F3" is the region at the N terminus of the extracellular domain of ErbB2 to which the 7C2 and/or 7F3 antibodies (each deposited with the ATCC, see below) bind. To screen for antibodies which bind to the 7C2/7F3 epitope, a routine cross-blocking assay such as that described in Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, Ed Harlow and David Lane (1988), can be performed. Alternatively, epitope mapping can be performed to establish whether the antibody binds to the 7C2/7F3 epitope on ErbB2 (e.g. any one or more of residues in the region from about residue 22 to about residue 53 of ErbB2; see Figure 4, and SEQ ID NO: 3). A tumor which "does not respond, or responds poorly, to treatment with a monoclonal anti-ErbB antibody" does not show statistically significant improvement in response to anti-ErbB antibody treatment when compared to no treatment or treatment with placebo in a recognized animal model or a human clinical trial, or which responds to initial treatment with anti-ErbB antibodies but grows as treatment is continued. A particularly suitable animal model for testing the efficacy of anti-ErbB antibodies is the transgenic animal model disclosed herein, and illustrated in Example 3. The terms "treat" or "treatment" refer to both therapeutic treatment and prophylactic or preventative measures, wherein the object is to prevent or slow down (lessen) an undesired physiological change or disorder, such as the development or spread of cancer. For purposes of this invention, beneficial or desired clinical results include, but are not limited to, alleviation of symptoms, diminishment of extent of disease, stabilized (i.e., not worsening) state of disease, delay or slowing of disease progression, amelioration or palliation of the disease state, and remission (whether partial or total), whether detectable or undetectable. "Treatment" can also mean prolonging survival as compared to expected survival if not receiving treatment. Those in need of treatment include those already with the condition or disorder as well as those prone to have the condition or disorder or those in which the condition or disorder is to be prevented. A "disorder" is any condition that would benefit from treatment of the present invention. This includes chronic and acute disorders or diseases including those pathological conditions which predispose the mammal to the disorder in question. Non-limiting examples of disorders to be treated herein include benign and malignant tumors; leukemias and lymphoid malignancies, in particular breast, ovarian, stomach, endometrial, salivary gland, lung, kidney, colon, thyroid, pancreatic, prostate or bladder cancer. A preferred disorder to be treated in accordance with the present invention is malignant tumor, such as breast cancer, that overexpresses an ErbB receptor (e.g. ErbB2 and/or EGFR), and does not respond or responds poorly to treatment with antibody to the receptor(s) that is/are overexpressed. A particularly preferred disorder is an ErbB2-overexpressing breast cancer that does not respond or responds poorly to HERCEPTIN ® therapy. The term "therapeutically effective amount" refers to an amount of a drug effective to treat a disease or disorder in a mammal. In the case of cancer, the therapeutically effective amount of the drug may reduce the number of cancer cells; reduce the tumor size; inhibit (i.e., slow to some extent and preferably stop) cancer cell infiltration into peripheral organs; inhibit (i.e., slow to some extent and preferably stop) tumor metastasis; inhibit, to some extent, tumor growth; and/or relieve to some extent one or more of the symptoms associated with the cancer. To the extent the drug may prevent growth and/or kill existing cancer cells, it may be cγtostatic and/or cγtotoxic. For cancer therapy, efficacy can, for example, be measured by assessing the time to disease progression (TTP) and/or determining the response rate (RR). The term "cytotoxic agent" as used herein refers to a substance that inhibits or prevents the function of cells and/or causes destruction of ceils. The term is intended to include radioactive isotopes (e.g. At 211 , 1 131 , 1 125 , Y 90 , Re 186 , Re 188 , Sm 153 , Bi 212 , P 32 and radioactive isotopes of Lu), chemotherapeutic agents, and toxins such as small molecule toxins or enzymatically active toxins of bacterial, fungal, plant or animal origin, including fragments and/or variants thereof. A "chemotherapeutic agent" is a chemical compound useful in the treatment of cancer. Examples of chemotherapeutic agents include alkylating agents such as thiotepa and cγclosphosphamide (CYTOXAN™); alkγl sulfonates such as busulfan, improsulfan and piposulfan; aziridines such as benzodopa, carboquone, meturedopa, and uredopa; ethylenimines and methylamelamines including altretamine, triethylenemelamine, trietylenephosphoramide, triethylenethiophosphaoramide and trimethγlolomelamine; nitrogen mustards such as chlorambucil, chlornaphazine, cholophosphamide, estramustine, ifosfamide, mechlorethamine, mechlorethamine oxide hydrochloride, melphalan, novembichin, phenesterine, prednimustine, trofosfamide, uracil mustard; nitrosureas such as carmustine, chlorozotocin, fotemustine, lomustine, nimustine, ranimustine; antibiotics such as aclacinomysins, actinomycin, authramycin, azaserine, bleomycins, cactinomycin, calicheamicin, carabicin, carminomycin, carzinophilin, chromomycins, dactinomycin, daunorubicin, detorubicin, 6-diazo-5-oxo-L-norleucine, doxorubicin, epirubicin, esorubicin, idarubicin, marcellomycin, mitomyciπs, mycophenolic acid, nogalamycin, olivomycins, peplomycin, potfiromycin, puromycin, quelamycin, rodorubicin, streptonigrin, streptozocin, tubercidin, ubenimex, zinostatin, zorubicin; anti- metabolites such as methotrexate and 5-fluorouracil (5-FU); folic acid analogues such as denopterin, methotrexate, pteropterin, trimetrexate; purine analogs such as fludarabine, 6-mercaptopurine, thiamipriπe, thioguanine; pyrimidine analogs such as ancitabine, azacitidine, 6-azauridine, carmofur, cγtarabine, dideoxyuridine, doxifluridine, enocitabine, fioxuridiπe, 5-FU; androgens such as caiusterone, dromostanolone propionate, epitiostanol, mepitiostane, testolactone; anti-adrenals such as aminoglutethimide, mitotane, trilostane; folic acid replenisher such as frolinic acid; aceglatone; aldophosphamide glycoside; aminolevulinic acid; amsacrine; bestrabucil; bisantreπe; edatraxate; defofamine; demecolcine; diaziquone; elfornithine; elliptinium acetate; etoglucid; gallium nitrate; hydroxyurea; lentinan; lonidamine; mitoguazone; mitoxantrone; mopidamol; nitracriπe; pentostatin; phenamet; pirarubicin; podophγllinic acid; 2- ethylhydrazide; procarbazine; PSK ; razoxane; sizofiran; spirogermanium; tenuazonic acid; triaziquone; 2, 2', 2' - trichlorotriethylamine; urethan; vindesine; dacarbazine; mannomustine; mitobronitol; mitolactol; pipobroman; gacytosine; arabinoside ("Ara-C"); cyclophosphamide; thiotepa; taxanes, e.g. paclitaxel (TAXOL * , Bristol-Myers Squibb Oncology, Princeton, NJ) and doxetaxel (TAXOTERE ® , Rhόne-Poulenc Rorer, Antony, France); chlorambucil; gemcitabine; 6-thioguanine; mercaptopurine; methotrexate; platinum analogs such as cisplatin and carboplatin; vinblastine; platinum; etoposide (VP-16); ifosfa ide; mitomycin C; itoxantrone; vincristine; vinorelbine; navelbine; novantrone; teniposide; daunomycin; aminopterin; xeloda; ibandronate; CPT-11; topoisomerase inhibitor RFS 2000; difluoromethylornithine (DMFO); retinoic acid; esperamicins; capecitabine; and pharmaceutically acceptable salts, acids or derivatives of any of the above. Also included in this definition are anti-hormonal agents that act to regulate or inhibit hormone action on tumors such as anti-estrogens including for example tamoxifen, raloxifene, aromatase inhibiting 4(5)-imidazoles, 4-hydroxytamoxifen, trioxifene, keoxifene, LY1 17018, onapristone, and toremifene (Fareston); and anti-androgens such as flutamide, nilutamide, bicalutamide, leuprolide, and goserelin; and pharmaceutically acceptable salts, acids or derivatives of any of the above. The term "prodrug" as used in this application refers to a precursor or derivative form of a pharmaceutically active substance that is less cytotoxic to tumor cells compared to the parent drug and is capable of being enzymatically activated or converted into the more active parent form. See, e.g., Wilman, "Prodrugs in Cancer Chemotherapy" Biochemical Society Transactions, 14, pp. 375-382, 615th Meeting Belfast (1986) and Stella et al., "Prodrugs: A Chemical Approach to Targeted Drug Delivery," Directed Drug Delivery, Borchardt et al, (ed.), pp. 247- 267, Humana Press (1985). The prodrugs of this invention include, but are not limited to, phosphate-containing prodrugs, thiophosphate-containing prodrugs, sulfate-containing prodrugs, peptide-containing prodrugs, D-amino acid- modified prodrugs, glycosγlated prodrugs, -lactam-containing prodrugs, optionally substituted phenoxyacetamide- containing prodrugs or optionally substituted phenylacetamide-containing prodrugs, 5-fluorocγtosine and other 5- fluorouridiπe prodrugs which can be converted into the more active cytotoxic free drug. Examples of cytotoxic drugs that can be derivatized into a prodrug form for use in this invention include, but are not limited to, those chemotherapeutic agents described above. The term "nucleic acid" refers to polγnucleotides such as deoxyribonucleic acid (DNA), and, where appropriate, ribonucleic acid (RNA). The term also includes, as equivalents, analogs of either DNA or RNA made from nucleotide analogs, and as applicable, single (sense or antisense) and double-stranded polynucleotides. An "isolated" nucleic acid molecule is a nucleic acid molecule that is identified and separated from at least one contaminant nucleic acid molecule with which it is ordinarily associated in the natural source of the nucleic acid. An isolated nucleic acid molecule is other than in the form or setting in which it is found in nature. Isolated nucleic acid molecules therefore are distinguished from the nucleic acid molecule as it exists in natural cells. However, an isolated nucleic acid molecule includes a nucleic acid molecule contained in cells that ordinarily express the antibody where, for example, the nucleic acid molecule is in a chromosomal location different from that of natural cells. As used herein, the term "vector" refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. The term "expression vector" includes plasmids, cosmids or phages capable of synthesizing the subject HER2 protein encoded by the respective recombinant gene carried by the vector. Preferred vectors are those capable of autonomous replication and/expression of nucleic acids to which they are linked. In the If present specification, "plasmid" and "vector" are used interchangeably, as the plasmid is the most commonly used form of vector. As used herein, the terms "transcriptional regulatory elements" and "transcriptional regulatory sequences" are used interchangeably and refer to nucleic acid, e.g. DNA sequences necessary for the expression of an operably linked coding sequence in a particular host organism. The control sequences that are suitable for prokarγotes, for example, include a promoter, optionally an operator sequence, and a ribosome binding site. Eukaryotic cells are known to utilize promoters, enhancers, splicing signals and polγadenylation signals. These terms are intended to encompass all elements that promote or regulate transcription, including promoters, core elements required for basic interaction of RNA polymerase and transcription factors, upstream elements, enhancers, and response elements (Lewin, "Genes V" (Oxford University Press, Oxford) pages 847-873). Reference herein to the transcriptional regulatory elements of a gene or class of gene includes both all or an intact region of the naturally occurring transcriptional regulatory elements and modified forms of the transcriptional regulatory elements of the gene or group of genes. Such modified forms include rearrangements of the elements, deletions of some elements or extraneous sequences, and insertion of heterologous elements. The modular nature of transcriptional regulatory elements and the absence of position- dependence of the function of some regulatory elements such as enhancers make such modifications possible. Numerous techniques are available for dissecting the regulatory elements of genes to determine their location and function. Such information can be used to direct modification of the elements, if desired. It is preferred, however, that an intact region of the transcriptional regulatory elements of a gene be used. The term "tissue-specific promoter" means a nucleotide sequence that serves as a promoter, i.e., regulates expression of a selected DNA sequence operably linked to the promoter, and which effects expression of the selected DNA sequence in specific cells of a tissue, such as cells of a mammary gland. In an illustrative embodiment, gene constructs utilizing mammary gland-specific promoters can be used to preferentially direct expression of a HER2 protein or protein fragment in the mammary gland tissue. Nucleic acid is "operably linked" when it is placed into a functional relationship with another nucleic acid sequence. For example, DNA for a presequence or secretory leader is operably linked to DNA for a polypeptide if it is expressed as a preprotein that participates in the secretion of the polypeptide; a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to facilitate translation. Generally, "operably linked" means that the DNA sequences being linked are contiguous, and, in the case of a secretory leader, contiguous and in reading phase. However, enhancers do not have to be contiguous. Linking is accomplished by ligation at convenient restriction sites. If such sites do not exist, the synthetic oligonucleotide adaptors or linkers are used in accordance with conventional practice. The term "transfection" refers to the introduction of a nucleic acid, e.g., an expression vector, into a recipient cell by nucleic acid-mediated gene transfer. "Transformation", as used herein, refers to a process in which a cell's genotype is changed as a result of the cellular uptake of exogenous DNA or RNA, and, for example, the transformed cell expresses a recombinant form of HER2. As used herein, the term "transgene" refers to a nucleic acid sequence which is partly or entirely heterologous, i.e., foreign, to the transgenic animal or cell into which it is introduced, or, is homologous to an endogenous gene of the transgenic animal or cell into which it is introduced, but which is designed to be inserted, or is inserted, into the animal's genome in such a way as to alter the genome of the cell into which it is inserted (e.g., it is inserted at a location which differs from that of the natural gene or its insertion results in a knockout). A transgene can be operably linked to one or more transcriptional regulatory sequences and any other nucleic acid, such as introns, that may be necessary for optimal expression of a selected nucleic acid. Accordingly, the term "transgene construct" refers to a nucleic acid which includes a transgene, and (optionally) such other nucleic acid sequences as transcriptionally regulatory sequence, polyadenγlation sites, replication origins, marker genes, etc., which may be useful in the general manipulation of the transgene for insertion in the genome of a host organism. The term "transgenic" is used herein as an adjective to describe the property, for example, of an animal or a construct, of harboring a transgene. For instance, as used herein, a "transgenic organism" is any animal, preferably a non-human mammal, in which one or more of the cells of the animal contain heterologous nucleic acid introduced by way of human intervention, such as by trangenic techniques well known in the art. The nucleic acid is introduced into the cell, directly or indirectly by introduction into a precursor of the cell, by way of deliberate genetic manipulation, such as by microinjection or by infection with a recombinant virus. The term genetic manipulation does not include classical cross-breeding, or in vitro fertilization, but rather is directed to the introduction of a recombinant DNA molecule. This molecule may be integrated within a chromosome, or it may be extrachromosomallγ replicating DNA. In the typical transgenic animals described herein, the transgene causes cells to express or overexpress a recombinant form of the subject HER2 proteins. The terms "founder line" and "founder animal" refer to those animals that are the mature product of the embryos to which the transgene was added, i.e., those animals that grew from the embryos into which DNA was inserted, and that were implanted into one or more surrogate hosts. The terms "progeny" and "progeny of the transgenic animal" refer to any and all offspring of every generation subsequent to the originally transformed mammals. The term "non-human mammal" refers to all members of the class Mammalia except humans. "Mammal" refers to any animal classified as a mammal, including humans, domestic and farm animals, and zoo, sports, or pet animals, such as mouse, rat, rabbit, pig, sheep, goat, cattle and higher primates. As used herein, the expressions "cell," "cell line," and "cell culture" are used interchangeably and all such designations include progeny. Thus, the words "transformants" and "transformed cells" include the primary subject cell and cultures derived therefrom without regard for the number of transfers. It is also understood that all progeny may not be precisely identical in DNA content, due to deliberate or inadvertent mutations. Mutant progeny that have the same function or biological activity as screened for in the originally transformed cell are included. Where distinct designations are intended, it will be clear from the context. A "liposome" is a small vesicle composed of various types of lipids, phospholipids and/or surfactant which is useful for delivery of a drug (such as the anti-ErbB2 antibodies disclosed herein and, optionally, a chemotherapeutic agent) to a mammal. The components of the liposome are commonly arranged in a bilayer formation, similar to the lipid arrangement of biological membranes. The term "package insert" is used to refer to instructions customarily included in commercial packages of therapeutic products, that contain information about the indications, usage, dosage, administration, contraindications and/or warnings concerning the use of such therapeutic products. A "cardioprotectant" is a compound or composition which prevents or reduces myocardial dysfunction (i.e. cardiomyopathy and/or congestive heart failure) associated with administration of a drug, such as an anti-ErbB antibody or its maytansinoid conjugate, to a patient. The cardioprotectant may, for example, block or reduce a free- radical-mediated cardiotoxic effect and/or prevent or reduce oxidative-stress injury. Examples of cardioprotectants encompassed by the present definition include the iron chelating agent dexrazoxane (ICRF-187) (Seifert et al. The Annals of Pharmacotherapy 28:1063-1072 (1994)); a lipid-lowering agent and/or anti-oxidant such as probucol (Singal etal. J. Mol. Cell Cardiol. 27:1055-1063 (1995)); amifostine (aminothiol 2-[(3-aminopropyl)amino]ethanethiol- dihydrogen phosphate ester, also called WR-2721, and the dephosphorylated cellular uptake form thereof called WR- 1065) and S-3-(3-methylaminopropylamino)propylphosphorothioic acid (WR-151327), see Green etal. Cancer Research 54:738-741 (1994); digoxin (Bristow, M.R. In: Bristow MR, ed. Drug-Induced Heart Disease. New York: Elsevier 191- 215 (1980)); beta blockers such as metoprolol (Hjalmarson etal. Z7w^ 47:Suppl 4:31-9 (1994); and Shaddy et al. Am. Heart J. 129:197-9 (1995)); vitamin E; ascorbic acid (vitamin C); free radical scavengers such as oleanolic acid, ursolic acid and N-acetylcysteine (NAC); spin trapping compounds such as alpha-phenyl-tert-butyl πitrone (PBN); (Paracchini et al., Anticancer Res. 13:1607-1612 (1993)); selenoorgaπic compounds such as P251 (Elbesen); and the like. 2. Detailed Description The present invention is based on results obtained in a novel murine HER2-transgenic tumor model in which HERCEPTIN ® or the murine antibody 4D5 from which HERCEPTIN ® was derived, had little effect on tumor growth. Using this model to test the efficacy of HERCEPTIN ® and HERCEPTIN ® -maytansinoid conjugates, it was surprisingly found that while the transplanted tumor obtained from such transgenic mice responded poorly to HERCEPTIN ® treatment, the HERCEPTIN ® -maytansinoid conjugates were highly efficacious. Accordingly, the present invention is based on the use of anti-ErbB antibody-maytansinoid conjugates in the treatment of ErbB overexpressing tumors that do not respond well to anti-ErbB antibody and/or maytansinoid treatment. A. Production of anti-ErbB antibodies A description follows as to exemplary techniques for the production of the antibodies used in accordance with the present invention. The production of antibodies will be illustrated with reference to aπti-ErbB2 antibodies but it will be apparent for those skilled in the art that antibodies to other members of the ErbB receptor family can be produced and modified in a similar manner. The ErbB2 antigen to be used for production of antibodies may be, e.g., a soluble form of the extracellular domain of ErbB2 or a portion thereof, containing the desired epitope. Alternatively, cells expressing ErbB2 at their cell surface (e.g. NIH-3T3 cells transformed to overexpress ErbB2; or a carcinoma cell line such as SK-BR-3 cells, see Stancovski et al. PNAS (USA) 88:8691 -8695 (1991)) can be used to generate antibodies. Other forms of ErbB2 useful for generating antibodies will be apparent to those skilled in the art. (i) Polyclonal antibodies Polyclonal antibodies are preferably raised in animals by multiple subcutaneous (sc) or iπtraperitoneal (ip) injections of the relevant antigen and an adjuvant. It may be useful to conjugate the relevant antigen to a protein that is immunogenic in the species to be immunized, e.g., keyhole limpet hemocyanin, serum albumin, bovine thyroglobulin, or soybean trypsin inhibitor using a bifunctional or derivatizing agent, for example, maleimidobenzoγl sulfosuccinimide ester (conjugation through cysteine residues), N-hydroxysuccinimide (through lysine residues), glutaraldehyde, succinic anhydride, SOCI 2 , or R N = C = NR, where R and R 1 are different alkyl groups. Animals are immunized against the antigen, immunogenic conjugates, or derivatives by combining, e.g., 100 g or 5 g of the protein or conjugate (for rabbits or mice, respectively) with 3 volumes of Freund's complete adjuvant and injecting the solution intradermally at multiple sites. One month later the animals are boosted with 1/5 to 1/10 the original amount of peptide or conjugate in Freund's complete adjuvant by subcutaneous injection at multiple sites. Seven to 14 days later the animals are bled and the serum is assayed for antibody titer. Animals are boosted until the titer plateaus. Preferably, the animal is boosted with the conjugate of the same antigen, but conjugated to a different protein and/or through a different cross-linking reagent. Conjugates also can be made in recombinant cell culture as protein fusions. Also, aggregating agents such as alum are suitably used to enhance the immune response. (ii) Monoclonal antibodies Monoclonal antibodies are obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally occurring mutations that may be present in minor amounts. Thus, the modifier "monoclonal" indicates the character of the antibody as not being a mixture of discrete antibodies. For example, the monoclonal antibodies may be made using the hybridoma method first described by Kohler et al. Nature, 256:495 (1975), or may be made by recombinant DNA methods (U.S. Patent No. 4,816,567). In the hybridoma method, a mouse or other appropriate host animal, such as a hamster, is immunized as hereinabove described to elicit lymphocytes that produce or are capable of producing antibodies that will specifically bind to the protein used for immunization. Alternatively, lymphocytes may be immunized in vitro. Lymphocytes then are fused with myeloma cells using a suitable fusing agent, such as polyethylene glycol, to form a hybridoma cell (Goding, Monoclonal Antibodies: Principles and Practice, pp.59-103 (Academic Press, 1986)). The hybridoma cells thus prepared are seeded and grown in a suitable culture medium that preferably contains one or more substances that inhibit the growth or survival of the unfused, parental myeloma cells. For example, if the parental myeloma cells lack the enzyme hypoxanthine guaniπe phosphoribosγl transferase (HGPRT or HPRT), the culture medium for the hybridomas typically will include hypoxanthine, aminopterin, and thymidine (HAT medium), which substances prevent the growth of HGPRT-deficient cells. Preferred myeloma cells are those that fuse efficiently, support stable high-level production of antibody by the selected antibody-producing cells, and are sensitive to a medium such as HAT medium. Among these, preferred myeloma cell lines are murine myeloma lines, such as those derived from MOPC-21 and MPC-11 mouse tumors available from the Salk Institute Cell Distribution Center, San Diego, California USA, and SP-2 or X63-Ag8-653 cells available from the American Type Culture Collection, Rockville, Maryland USA. Human myeloma and mouse-human heteromyeloma cell lines also have been described for the production of human monoclonal antibodies (Kozbor, J. Immunol, 133:3001 (1984); and Brodeur et al, Monoclonal Antibody Production Techniques and Applications, pp. 51- 63 (Marcel Dekker, Inc., New York, 1987)). Culture medium in which hybridoma cells are growing is assayed for production of monoclonal antibodies directed against the antigen. Preferably, the binding specificity of monoclonal antibodies produced by hybridoma cells is determined by immunoprecipitatioπ or by an in vitro binding assay, such as radioimmunoassay (RIA) or enzyme- linked immunoabsorbent assay (ELISA). The binding affinity of the monoclonal antibody can, for example, be determined by the Scatchard analysis of Muπson et al, Anal. Biochem., 107:220 (1980). After hybridoma cells are identified that produce antibodies of the desired specificity, affinity, and/or activity, the clones may be subcloned by limiting dilution procedures and grown by standard methods (Goding, Monoclonal Antibodies: Principles and Practice, pp.59-103 (Academic Press, 1986)). Suitable culture media for this purpose include, for example, D-MEM or RPMI-1640 medium. In addition, the hybridoma cells may be grown in vivo as ascites tumors in an animal. The monoclonal antibodies secreted by the subclones are suitably separated from the culture medium, ascites fluid, or serum by conventional antibody purification procedures such as, for example, protein A-Sepharose, hydroxylapatite chromatography, gel electrophoresis, dialysis, or affinity chromatography. DNA encoding the monoclonal antibodies is readily isolated and sequenced using conventional procedures (e.g., by using oligonucleotide probes that are capable of binding specifically to genes encoding the heavy and light chains of murine antibodies). The hybridoma cells serve as a preferred source of such DNA. Once isolated, the DNA may be placed into expression vectors, which are then transfected into host cells such as £ coli cells, simian COS cells, Chinese Hamster Ovary (CHO) cells, or myeloma cells that do not otherwise produce antibody protein, to obtain the synthesis of monoclonal antibodies in the recombinant host cells. Review articles on recombinant expression in bacteria of DNA encoding the antibody include Skerra et al, Curr. Opinion in Immunol, 5:256-262 (1993) and Plϋckthun, Immunol. Revs., 130:151-188 (1992). In a further embodiment, monoclonal antibodies or antibody fragments can be isolated from antibody phage libraries generated using the techniques described in McCafferty et al. Nature, 348:552-554 (1990). Clackson et al, Nature, 352:624-628 (1991 ) and Marks et al., J. Mol. Biol, 222:581-597 (1991) describe the isolation of murine and human antibodies, respectively, using phage libraries. Subsequent publications describe the production of high affinity (nM range) human antibodies by chain shuffling (Marks et al, Bio/Technology, 10:779-783 (1992)), as well as combinatorial infection and //? vivo recombination as a strategy for constructing very large phage libraries (Waterhouse etal, Nuc. Acids. Res., 21:2265-2266 (1993)). Thus, these techniques are viable alternatives to traditional monoclonal antibody hybridoma techniques for isolation of monoclonal antibodies. The DNA also may be modified, for example, by substituting the coding sequence for human heavy chain and light chain constant domains in place of the homologous murine sequences (U.S. Patent No. 4,816,567; and Morrison, etal, Proc. Natl Acad. Sci. USA, 81:6851 (1984)), or by covalently joining to the immunoglobuiin coding sequence all or part of the coding sequence for a non-immunoglobuliπ polypeptide. Typically such non-immuπoglobulin polypeptides are substituted for the constant domains of an antibody, or they are substituted for the variable domains of one antigen-combining site of an antibody to create a chimeric bivalent antibody comprising one antigen-combining site having specificity for an antigen and another antigen- combining site having specificity for a different antigen. (iii) Humanized antibodies Methods for humanizing non-human antibodies have been described in the art. Preferably, a humanized antibody has one or more amino acid residues introduced into it from a source which is non-human. These non-human amino acid residues are often referred to as "import" residues, which are typically taken from an "import" variable domain. Humanizatioπ can be essentially performed following the method of Winter and co-workers (Jones et al, Nature, 321 :522-525 (1986); Riechmann et al., Nature, 332:323-327 (1988); Verhoeyen et al, Science, 239:1534- 1536 (1988)), by substituting hypervariable region sequences for the corresponding sequences of a human antibody. Accordingly, such "humanized" antibodies are chimeric antibodies (U.S. Patent No. 4,816,567) wherein substantially less than an intact human variable domain has been substituted by the corresponding sequence from a non-human species. In practice, humanized antibodies are typically human antibodies in which some hypervariable region residues and possibly some FR residues are substituted by residues from analogous sites in rodent antibodies. The choice of human variable domains, both light and heavy, to be used in making the humanized antibodies is very important to reduce antigenicity. According to the so-called "best-fit" method, the sequence of the variable domain of a rodent antibody is screened against the entire library of known human variable-domain sequences. The human sequence which is closest to that of the rodent is then accepted as the human framework region (FR) for the humanized antibody (Sims et al, J. Immunol, 151 :2296 (1993); Chothia et al, J. Mol. Biol, 196:901 (1987)). Another method uses a particular framework region derived from the consensus sequence of all human antibodies of a particular subgroup of light or heavy chains. The same framework may be used for several different humanized antibodies (Carter etal, Proc. Natl. Acad. Sci. USA, 89:4285 (1992); Presta etal, J. Immunol, 151:2623 (1993)). It is further important that antibodies be humanized with retention of high affinity for the antigen and other favorable biological properties. To achieve this goal, according to a preferred method, humanized antibodies are prepared by a process of analysis of the parental sequences and various conceptual humanized products using three- dimensional models of the parental and humanized sequences. Three-dimensional immunoglobuiin models are commonly available and are familiar to those skilled in the art. Computer programs are available which illustrate and display probable three-dimensional conformational structures of selected candidate immunoglobuiin sequences. Inspection of these displays permits analysis of the likely role of the residues in the functioning of the candidate immunoglobuiin sequence, i.e., the analysis of residues that influence the ability of the candidate immunoglobuiin to bind its antigen. In this way, FR residues can be selected and combined from the recipient and import sequences so that the desired antibody characteristic, such as increased affinity for the target antigen(s), is achieved. In general, the hypervariable region residues are directly and most substantially involved in influencing antigen binding. Example 1 below describes production of an exemplary humanized anti-ErbB2 antibody. The humanized antibody herein may, for example, comprise nonhuman hypervariable region residues incorporated into a human variable heavy domain and may further comprise a framework region (FR) substitution at a position selected from the group consisting of 69H, 71 H and 73H utilizing the variable domain numbering system set forth in Kabat et al. Sequences of Proteins of Immunological Interest, 5th Ed. Public Health Service, National Institutes of Health, Bethesda, MD (1991 ). In one embodiment, the humanized antibody comprises FR substitutions at two or all of positions 69H, 71 H and 73H. Various forms of the humanized antibody are contemplated. For example, the humanized antibody may be an antibody fragment, such as a Fab. Alternatively, the humanized antibody may be an intact antibody, such as an intact IgGI antibody. (iv) Human antibodies As an alternative to humanization, human antibodies can be generated. For example, it is now possible to produce transgenic animals (e.g., mice) that are capable, upon immunization, of producing a full repertoire of human antibodies in the absence of endogenous immunoglobuiin production. For example, it has been described that the homozygous deletion of the antibody heavy-chain joining region (J H ) gene in chimeric and germ-line mutant mice results in complete inhibition of endogenous antibody production. Transfer of the human germ-line immunoglobuiin gene array in such germ-line mutant mice will result in the production of human antibodies upon antigen challenge. See, e.g., Jakobovits et al, Proc. Natl. Acad. Sci. USA, 90:2551 (1993); Jakobovits etal. Nature, 362:255-258 (1993); Bruggermann et al. Year in Immuno., 7:33 (1993); and U.S. Patent Nos. 5,591,669, 5,589,369 and 5,545,807. Alternatively, phage display technology (McCafferty et al, Nature 348:552-553 (1990)) can be used to produce human antibodies and antibody fragments in vitro, from immunoglobuiin variable (V) domain gene repertoires from unimmunized donors. According to this technique, antibody V domain genes are cloned in-frame into either a major or minor coat protein gene of a filamentous bacteriophage, such as M13 or fd, and displayed as functional antibody fragments on the surface of the phage particle. Because the filamentous particle contains a single-stranded DNA copy of the phage genome, selections based on the functional properties of the antibody also result in selection of the gene encoding the antibody exhibiting those properties. Thus, the phage mimics some of the properties of the B-cell. Phage display can be performed in a variety of formats; for their review see, e.g., Johnson, Kevin S. and Chiswell, David J., Current Opinion in Structural Biology 3:564-571 (1993). Several sources of V-gene segments can be used for phage display. Clackson et al, Nature, 352:624-628 (1991) isolated a diverse array of anti-oxazolone antibodies from a small random combinatorial library of V genes derived from the spleens of immunized mice. A repertoire of V genes from unimmunized human donors can be constructed and antibodies to a diverse array of antigens (including self-antigens) can be isolated essentially following the techniques described by Marks et a/., J Mol. Biol. 222:581-597 (1991), or Griffith etal, EMBOJ. 12:725-734 (1993). See, also, U.S. Patent Nos. 5,565,332 and 5,573,905. As discussed above, human antibodies may also be generated by in vitro activated B cells (see U.S. Patents 5,567,610 and 5,229,275). Human anti-ErbB2 antibodies are described in U.S. Patent No. 5,772,997 issued June 30, 1998 and WO 97/00271 published January 3, 1997. (v) Antibody fragments Various techniques have been developed for the production of antibody fragments. Traditionally, these fragments were derived via proteolytic digestion of intact antibodies (see, e.g., Morimoto et al. , Journal of Biochemical and Biophysical Methods 24:107-117 (1992); and Brennan et al, Science, 229:81 (1985)). However, these fragments can now be produced directly by recombinant host cells. For example, the antibody fragments can be isolated from the antibody phage libraries discussed above. Alternatively, Fab'-SH fragments can be directly recovered from £. cσ// and chemically coupled to form F(ab') 2 fragments (Carter et a/., Bio/Technology 10:163-167 (1992)). According to another approach, F(ab') 2 fragments can be isolated directly from recombinant host cell culture. Other techniques for the production of antibody fragments will be apparent to the skilled practitioner. In other embodiments, the antibody of choice is a single chain Fv fragment (scFv). See WO 93/16185; U.S. Patent No. 5,571,894; and U.S. Patent No. 5,587,458. The antibody fragment may also be a "linear antibody", e.g., as described in U.S. Patent 5,641 ,870 for example. Such linear antibody fragments may be monospecific or bispecific. (vi) Bispecific antibodies Bispecific antibodies are antibodies that have binding specificities for at least two different epitopes. Exemplary bispecific antibodies may bind to two different epitopes of the ErbB2 protein. Other such antibodies may combine an ErbB2 binding site with binding site(s) for EGFR, ErbB3 and/or ErbB4. Alternatively, an anti-ErbB2 arm may be combined with an arm which binds to a triggering molecule on a leukocyte such as a T-cell receptor molecule (e.g. CD2 or CD3), or Fc receptors for IgG (Fc R), such as Fc Rl (CD64), Fc Rll (CD32) and Fc Rill (CD16) so as to focus cellular defense mechanisms to the ErbB2-expressing cell. Bispecific antibodies may also be used to localize cytotoxic agents to cells which express ErbB2. WO 96/16673 describes a bispecific anti ErbB2/antι Fc Rill antibody and U.S. Patent No. 5,837,234 discloses a bispecific anti ErbB2/antι Fc Rl antibody. A bispecific anti ErbB2/Fc antibody is shown in W098/02463. U.S. Patent No. 5,821,337 teaches a bispecific anti ErbB2/antι CD3 antibody. Methods for making bispecific antibodies are known in the art Traditional production of full length bispecific antibodies is based on the coexpression of two immunoglobuiin heavy chain light chain pairs, where the two chains have different specificities (Millstein et al, Nature, 305:537 539 (1983)). Because of the random assortment of immunoglobuiin heavy and light chains, these hybπdomas (quadromas) produce a potential mixture of 10 different antibody molecules, of which only one has the correct bispecific structure. Purification of the correct molecule, which is usually done by affinity chromatography steps, is rather cumbersome, and the product yields are low. Similar procedures are disclosed in WO 93/08829, and in Traunecker et al, EMBOJ., 10:3655 3659 (1991 ) According to a different approach, antibody variable domains with the desired binding specificities (antibody-antigen combining sites) are fused to immunoglobuiin constant domain sequences. The fusion preferably is with an immunoglobuiin heavy chain constant domain, comprising at least part of the hinge, CH2, and CH3 regions It is preferred to have the first heavy chain constant region (CH1 ) containing the site necessary for light chain binding, present in at least one of the fusions. DNAs encoding the immunoglobuiin heavy chain fusions and, if desired, the immunoglobuiin light chain, are inserted into separate expression vectors, and are co-transfected into a suitable host organism. This provides for great flexibility in adjusting the mutual proportions of the three polypeptide fragments in embodiments when unequal ratios of the three polypeptide chains used in the construction provide the optimum yields. It is, however, possible to insert the coding sequences for two or all three polypeptide chains in one expression vector when the expression of at least two polypeptide chains in equal ratios results in high yields or when the ratios are of no particular significance. In a preferred embodiment of this approach, the bispecific antibodies are composed of a hybrid immunoglobuiin heavy chain with a first binding specificity in one arm, and a hybrid immunoglobuiin heavy chain light chain pair (providing a second binding specificity) in the other arm. It was found that this asymmetric structure facilitates the separation of the desired bispecific compound from unwanted immunoglobuiin chain combinations, as the presence of an immunoglobuiin light chain in only one half of the bispecific molecule provides for a facile way of separation This approach is disclosed in WO 94/04690. For further details of generating bispecific antibodies see, for example, Suresh et al, Methods in Enzy ology, 121 :210 (1986). According to another approach described in U S. Patent No. 5,731,168, the interface between a pair of antibody molecules can be engineered to maximize the percentage of heterodimers which are recovered from recombinant cell culture. The preferred interface comprises at least a part of the C H 3 domain of an antibody constant domain. In this method, one or more small ammo acid side chains from the interface of the first antibody molecule are replaced with larger side chains (e.g. tyrosine or tryptophan). Compensatory "cavities" of identical or similar size to the large side chaιn(s) are created on the interface of the second antibody molecule by replacing large ammo acid side chains with smaller ones (e.g. alamne or threonine). This provides a mechanism for increasing the yield of the heterodimer over other unwanted end-products such as homodimers Techniques for generating bispecific antibodies from antibody fragments have also been described in the literature. For example, bispecific antibodies can be prepared using chemical linkage. Brennan et al. Science, 229: 81 (1985) describe a procedure wherein intact antibodies are proteolγtically cleaved to generate F(ab') 2 fragments. These fragments are reduced in the presence of the dithiol complexing agent sodium arsenite to stabilize vicinal dithiols and prevent intermolecular disulfide formation. The Fab' fragments generated are then converted to thionitrobenzoate (TNB) derivatives. One of the Fab'-TNB derivatives is then reconverted to the Fab'-thiol by reduction with ercaptoethylamine and is mixed with an equimolar amount of the other Fab'-TNB derivative to form the bispecific antibody. The bispecific antibodies produced can be used as agents for the selective immobilization of enzymes. Recent progress has facilitated the direct recovery of Fab'-SH fragments from £ coli, which can be chemically coupled to form bispecific antibodies. Shalaby et al, J. Exp. Med., 175: 217-225 (1992) describe the production of a fully humanized bispecific antibody F(ab') 2 molecule. Each Fab' fragment was separately secreted from £ coli and subjected to directed chemical coupling in vitro to form the bispecific antibody. The bispecific antibody thus formed was able to bind to cells overexpressing the ErbB2 receptor and normal human T cells, as well as trigger the lytic activity of human cytotoxic lymphocytes against human breast tumor targets. Various techniques for making and isolating bispecific antibody fragments directly from recombinant cell culture have also been described. For example, bispecific antibodies have been produced using leucine zippers. Kostelny et al, J. Immunol, 148(5):1547-1553 (1992). The leucine zipper peptides from the Fos and Jun proteins were linked to the Fab' portions of two different antibodies by gene fusion. The antibody homodimers were reduced at the hinge region to form monomers and then re-oxidized to form the antibody heterodimers. This method can also be utilized for the production of antibody homodimers. The "diabody" technology described by Hollinger et al, Proc. Natl. Acad. Sci. USA, 90:6444-6448 (1993) has provided an alternative mechanism for making bispecific antibody fragments. The fragments comprise a heavy-chain variable domain (V H ) connected to a light-chain variable domain (V L ) by a linker which is too short to allow pairing between the two domains on the same chain. Accordingly, the V H and V L domains of one fragment are forced to pair with the complementary V L and V H domains of another fragment, thereby forming two antigen-binding sites. Another strategy for making bispecific antibody fragments by the use of single- chain Fv (sFv) dimers has also been reported. See Gruber et al, J. Immunol, 152:5368 (1994). Antibodies with more than two valencies are contemplated. For example, trispecific antibodies can be prepared. Tutt etal. J. Immunol. 147: 60 (1991). (vii) Other amino acid sequence modifications Amino acid sequence modification(s) of the anti-ErbB2 antibodies described herein are contemplated. For example, it may be desirable to improve the binding affinity and/or other biological properties of the antibody. Amino acid sequence variants of the anti-ErbB2 antibody are prepared by introducing appropriate nucleotide changes into the anti-ErbB2 antibody nucleic acid, or by peptide synthesis. Such modifications include, for example, deletions from, and/or insertions into and/or substitutions of, residues within the amino acid sequences of the anti-ErbB2 antibody. Any combination of deletion, insertion, and substitution is made to arrive at the final construct, provided that the final construct possesses the desired characteristics. The amino acid changes also may alter post-translational processes of the anti ErbB2 antibody, such as changing the number or position of glycosylation sites. A useful method for identification of certain residues or regions of the anti-ErbB2 antibody that are preferred locations for mutagenesis is called "alanine scanning mutagenesis" as described by Cunningham and Wells Science, 244:1081-1085 (1989). Here, a residue or group of target residues are identified (e.g., charged residues such as arg, asp, his, lys, and glu) and replaced by a neutral or negatively charged amino acid (most preferably alanine or polyalaniπe) to affect the interaction of the amino acids with ErbB2 antigen. Those amino acid locations demonstrating functional sensitivity to the substitutions then are refined by introducing further or other variants at, or for, the sites of substitution. Thus, while the site for introducing an amino acid sequence variation is predetermined, the nature of the mutation perse need not be predetermined. For example, to analyze the performance of a mutation at a given site, ala scanning or random mutagenesis is conducted at the target codoπ or region and the expressed anti- ErbB2 antibody variants are screened for the desired activity. Amino acid sequence insertions include amino- and/or carboxyl-terminal fusions ranging in length from one residue to polypeptides containing a hundred or more residues, as well as intrasεquence insertions of single or multiple amino acid residues. Examples of terminal insertions include an anti-ErbB2 antibody with an N-terminal methionyl residue or the antibody fused to a cytotoxic polypeptide. Other insertional variants of the anti-ErbB2 antibody molecule include the fusion to the N- or C-terminus of the anti-ErbB2 antibody to an enzyme (e.g. for ADEPT) or a polypeptide which increases the serum half-life of the antibody. Another type of variant is an amino acid substitution variant. These variants have at least one amino acid residue in the anti-ErbB2 antibody molecule replaced by a different residue. The sites of greatest interest for substitutional mutagenesis include the hypervariable regions, but FR alterations are also contemplated. Conservative substitutions are shown in Table 1 under the heading of "preferred substitutions". If such substitutions result in a change in biological activity, then more substantial changes, denominated "exemplary substitutions" in Table 1 , or as further described below in reference to amino acid classes, may be introduced and the products screened. Table 1 Val (V) ile; leu; met; phe; leu ala; norleucine Substantial modifications in the biological properties of the antibody are accomplished by selecting substitutions that differ significantly in their effect on maintaining (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a sheet or helical conformation, (b) the charge or hγdrophobicity of the molecule at the target site, or (c) the bulk of the side chain. Naturally occurring residues are divided into groups based on common side chain properties: (1 ) hydrophobic: norleucine, met, ala, val, leu, lie; (2) neutral hydrophihc: cys, ser, thr; (3) acidic: asp, glu; (4) basic: asn, gin, his, lys, arg; (5) residues that influence chain orientation: gly, pro; and (6) aromatic: trp, tyr, phe. Non-conservative substitutions will entail exchanging a member of one of these classes for another class. Any cysteine residue not involved in maintaining the proper conformation of the antι-ErbB2 antibody also may be substituted, generally with seπne, to improve the oxidative stability of the molecule and prevent aberrant crosslinking. Conversely, cysteine bond(s) may be added to the antibody to improve its stability (particularly where the antibody is an antibody fragment such as an Fv fragment). A particularly preferred type of substitutional variant involves substituting one or more hypervariable region residues of a parent antibody (e.g. a humanized or human antibody). Generally, the resulting varιant(s) selected for further development will have improved biological properties relative to the parent antibody from which they are generated. A convenient way for generating such substitutional variants involves affinity maturation using phage display. Briefly, several hypervariable region sites (e.g. 6 7 sites) are mutated to generate all possible ammo substitutions at each site. The antibody variants thus generated are displayed in a monovalent fashion from filamentous phage particles as fusions to the gene III product of M13 packaged within each particle. The phage displayed variants are then screened for their biological activity (e.g. binding affinity) as herein disclosed. In order to identify candidate hypervariable region sites for modification, alanine scanning mutagenesis can be performed to identify hypervariable region residues contributing significantly to antigen binding. Alternatively, or additionally, it may be beneficial to analyze a crystal structure of the antigen-antibody complex to identify contact points between the antibody and human ErbB2. Such contact residues and neighboring residues are candidates for substitution according to the techniques elaborated herein. Once such variants are generated, the panel of variants is subjected to screening as described herein and antibodies with superior properties in one or more relevant assays may be selected for further development. It may be desirable to modify the antibody of the invention with respect to effector function, e.g. so as to enhance antigen-dependent cell-mediated cyotoxicity (ADCC) and/or complement dependent cytotoxicity (CDC) of the antibody. This may be achieved by introducing one or more amino acid substitutions in an Fc region of the antibody. Alternatively or additionally, cysteine residue(s) may be introduced in the Fc region, thereby allowing interchain disulfide bond formation in this region. The homodimeric antibody thus generated may have improved internalization capability and/or increased complement-mediated cell killing and antibody-dependent cellular cytotoxicity (ADCC). See Caron etal. J. Exp Med. 176:1191 -1195 (1992) and Shopes, B. J Immunol. 148:2918-2922 (1992). Homodimeric antibodies with enhanced anti-tumor activity may also be prepared using heterobifunctional cross-linkers as described in Wolff et al. Cancer Research 53:2560-2565 (1993). Alternatively, an antibody can be engineered which has dual Fc regions and may thereby have enhanced complement lysis and ADCC capabilities. See Stevenson et al. Anti-Cancer Drug Design 3:219-230 (1989). To increase the serum half life of the antibody, one may incorporate a salvage receptor binding epitope into the antibody (especially an antibody fragment) as described in U.S. Patent 5,739,277, for example. As used herein, the term "salvage receptor binding epitope" refers to an epitope of the Fc region of an IgG molecule (e.g., IgG,, lgG 2 , lgG 3 , or lgG 4 ) that is responsible for increasing the in vivo serum half-life of the IgG molecule. (mi) Glycosylation Variants Antibodies are glycosylated at conserved positions in their constant regions (Jefferis and Lund, Chem. Immunol. 65:111-128 [1997]; Wright and Morrison, TibTECH 15:26-32 [1997]). The oligosaccharide side chains of the immunoglobulins affect the protein's function (Boyd et al, Mol. Immunol. 32:131 1-1318 [19961; Wittwe and Howard, Biochem. 29:4175-4180 [1990]), and the intramolecular interaction between portions of the glycoprotein which can affect the conformation and presented three-dimensional surface of the glycoprotein (Hefferis and Lund, supra; Wyss and Wagner, Current Opin. Biotech. 7:409-416 [19961). Oligosaccharides may also serve to target a given glycoprotein to certain molecules based upon specific recognition structures. For example, it has been reported that in agalactosylated IgG, the oligosaccharide moiety 'flips' out of the inter-CH2 space and terminal N- acetylglucosamine residues become available to bind mannose binding protein (Malhotra et al, Nature Med. 1:237- 243 [1995]). Removal by glycopeptidase of the oligosaccharides from CAMPATH-1 H (a recombinant humanized murine monoclonal IgGI antibody which recognizes the CDw52 antigen of human lymphocytes) produced in Chinese Hamster Ovary (CHO) cells resulted in a complete reduction in complement mediated lysis (CMCL) (Boyd et al, Mol. Immunol. 32:1311-1318 [19961), while selective removal of sialic acid residues using neuraminidase resulted in no loss of DMCL. Glycosylation of antibodies has also been reported to affect antibody-dependent cellular cytotoxicity (ADCC). In particular, CHO cells with tetracycliπe-regulated expression of β(1 ,4)-N-acetylglucosaminyltransferase III (GnTIII), a glycosyltransferase catalyzing formation of bisecting GlcNAc, was reported to have improved ADCC activity (Umana etal, Mature Biotech. 17:176-180 [1999]). Glycosylation of antibodies is typically either N-linked or 0-linkεd. N-linked refers to the attachment of the carbohydrate moiety to the side chain of an asparagine residue. The tripeptide sequences asparagine-X-serine and asparagine-X-threonine, where X is any amino acid except proline, are the recognition sequences for enzymatic attachment of the carbohydrate moiety to the asparagine side chain. Thus, the presence of either of these tripeptide sequences in a polypeptide creates a potential glycosylation site. O-linked glycosylation refers to the attachment of one of the sugars N-aceγlgalactosamine, galactose, or xylose to a hydroxyamino acid, most commonly serine or threonine, although 5-hydroxyproline or 5-hydroxγlysine may also be used. Glycosylation variants of antibodies are variants in which the glycosylation pattern of an antibody is altered. By altering is meant deleting one or more carbohydrate moieties found in the antibody, adding one or more carbohydrate moieties to the antibody, changing the composition of glycosylation (glycosylation pattern), the extent of glycosylation, etc. Addition of glycosylation sites to the antibody is conveniently accomplished by altering the amino acid sequence such that it contains one or more of the above-described tripeptide sequences (for N-linked glycosylation sites). The alteration may also be made by the addition of, or substitution by, one or more serine or threonine residues to the sequence of the original antibody (for O-linked glycosylation sites). Similarly, removal of glycosylation sites can be accomplished by amino acid alteration within the native glycosylation sites of the antibody. The amino acid sequence is usually altered by altering the underlying nucleic acid sequence. Nucleic acid molecules encoding amino acid sequence variants of the anti-ErbB2 antibody are prepared by a variety of methods known in the art. These methods include, but are not limited to, isolation from a natural source (in the case of naturally occurring amino acid sequence variants) or preparation by oligonucieotide-mediated (or site-directed) mutagenesis, PCR mutagenesis, and cassette mutagenesis of an earlier prepared variant or a non-variant version of the anti-ErbB2 antibody. The glycosylation (including glycosylation pattern) of antibodies may also be altered without altering the amino acid sequence or the underlying nucleotide sequence. Glycosylation largely depends on the host cell used to express the antibody. Since the cell type used for expression of recombinant glycoproteins, e.g. antibodies, as potential therapeutics is rarely the native cell, significant variations in the glycosylation pattern of the antibodies can be expected (see, e.g. Hse et al, J. Biol. Chem. 272:9062-9070 [1997]). In addition to the choice of host cells, factors which affect glycosylation during recombinant production of antibodies include growth mode, media formulation, culture density, oxygenation, pH, purification schemes and the like. Various methods have been proposed to alter the glycosylation pattern achieved in a particular host organism including introducing or overexpressing certain enzymes involved in oligosaccharide production (U. S. Patent Nos. 5,047,335; 5,510,261 and 5.278,299). Glycosylation, or certain types of glycosylation, can be enzymatically removed from the glycoprotein, for example using endoglycosidase H (Endo H). In addition, the recombinant host cell can be genetically engineered, e.g. make defective in processing certain types of polysaccharides. These and similar techniques are well known in the art. The glycosylation structure of antibodies can be readily analyzed by conventional techniques of carbohydrate analysis, including lectin chromatography, NMR, Mass spectrometry, HPLC, GPC, monosaccharide compositional analysis, sequential enzymatic digestion, and HPAEC-PAD, which uses high pH anion exchange chromatography to separate oligosaccharides based on charge. Methods for releasing oligosaccharides for analytical purposes are also known, and include, without limitation, enzymatic treatment (commonly performed using peptide-N- glycosidase F/endo-β-galactosidase), elimination using harsh alkaline environment to release mainly O-linked structures, and chemical methods using anhydrous hydrazine to release both N- and O-linked oligosaccharides. (viii) Screening for antibodies with the desired properties Techniques for generating antibodies have been described above. One may further select antibodies with certain biological characteristics, as desired. For example, to identify growth inhibitory anti-ErbB2 antibodies, one may screen for antibodies which inhibit the growth of cancer cells which overexpress ErbB2. In one embodiment, the growth inhibitory antibody of choice is able to inhibit growth of SK-BR-3 cells in cell culture by about 20-100% and preferably by about 50-100% at an antibody concentration of about 0.5 to 30 g/ml. To identify such antibodies, the SK-BR-3 assay described in U.S. Patent No. 5,677,171 can be performed. According to this assay, SK-BR-3 cells are grown in a 1:1 mixture of F12 and DMEM medium supplemented with 10% fetal bovine serum, glutamine and penicillin streptomycin. The SK-BR-3 cells are plated at 20,000 cells in a 35mm cell culture dish (2mls/35mm dish). 0.5 to 30 g/ml of the anti-ErbB2 antibody is added per dish. After six days, the number of cells, compared to untreated cells are counted using an electronic COULTER cell counter. Those antibodies which inhibit growth of the SK-BR-3 cells by about 20-100% or about 50-100% may be selected as growth inhibitory antibodies. To select for antibodies which induce cell death, loss of membrane integrity as indicated by, e.g., PI, trypan blue or 7AAD uptake may be assessed relative to control. The preferred assay is the PI uptake assay using BT474 cells. According to this assay, BT474 cells (which can be obtained from the American Type Culture Collection (Rockville, MD)) are cultured in Dulbecco's Modified Eagle Medium (D-MEM):Ham's F-12 (50:50) supplemented with 10% heat-inactivated FBS (Hyclone) and 2 mM L-glutamine. (Thus, the assay is performed in the absence of complement and immune effector cells). The BT474 cells are seeded at a density of 3 x I O 6 per dish in 100 x 20 mm dishes and allowed to attach overnight. The medium is then removed and replaced with fresh medium alone or medium containing 10 g/ml of the appropriate monoclonal antibody. The cells are incubated for a 3 day time period. Following each treatment, monolayers are washed with PBS and detached by trypsinization. Cells are then centrifuged at 1200rpm for 5 minutes at 4°C, the pellet resuspended in 3 ml ice cold Ca 2+ binding buffer (10 mM Hepes, pH 7.4, 140 mM NaCI, 2.5 mM CaCI 2 ) and aliquoted into 35 mm strainer-capped 12 x 75 tubes (1ml per tube, 3 tubes per treatment group) for removal of cell clumps. Tubes then receive PI (10 g/ml). Samples may be analyzed using a FACSCAN flow cytometer and FACSCONVERT CellQuest software (Becton Dickinson). Those antibodies which induce statistically significant levels of cell death as determined by PI uptake may be selected as cell death- inducing antibodies. In order to select for antibodies which induce apoptosis, an annexin binding assay using BT474 cells is available. The BT474 cells are cultured and seeded in dishes as discussed in the preceding paragraph. The medium is then removed and replaced with fresh medium alone or medium containing 10 g/ml of the monoclonal antibody. Following a three day incubation period, monolayers are washed with PBS and detached by trypsinization. Cells are then centrifuged, resuspended in Ca 2+ binding buffer and aliquoted into tubes as discussed above for the cell death assay. Tubes then receive labeled annexin (e.g. annexin V-FTIC) (1 g/ml). Samples may be analyzed using a FACSCAN flow cytometer and FACSCONVERT CellQuest software (Becton Dickinson). Those antibodies which induce statistically significant levels of annexin binding relative to control are selected as apoptosis-inducing antibodies. In addition to the annexin binding assay, a DNA staining assay using BT474 cells is available. In order to perform this assay, BT474 cells which have been treated with the antibody of interest as described in the preceding two paragraphs are incubated with 9 g/ml HOECHST 33342 for 2 hr at 37°C, then analyzed on an EPICS ELITE flow cytometer (Coulter Corporation) using MODFIT LT software (Verity Software House). Antibodies which induce a change in the percentage of apoptotic cells which is 2 fold or greater (and preferably 3 fold or greater) than untreated cells (up to 100% apoptotic cells) may be selected as pro-apoptotic antibodies using this assay. To identify an antibody which blocks ligand activation of an ErbB receptor, the ability of the antibody to block ErbB ligand binding to cells expressing the ErbB receptor (e.g. in conjugation with another ErbB receptor with which the ErbB receptor of interest forms an ErbB hetero-oligomer) may be determined. For example, cells naturally expressing, or transfected to express, ErbB receptors of the ErbB hetero-oligomer may be incubated with the antibody and then exposed to labeled ErbB ligand. The ability of the anti-ErbB2 antibody to block ligand binding to the ErbB receptor in the ErbB hetero-oligomer may then be evaluated. For example, inhibition of HRG binding to MCF7 breast tumor cell lines by anti-ErbB2 antibodies may be performed using monolayer MCF7 cultures on ice in a 24-well-plate format essentially as described in Example 1 below. Anti-ErbB2 monoclonal antibodies may be added to each well and incubated for 30 minutes. 125 l-labeled rHRG 1 , 77 . 224 (25 pm) may then be added, and the incubation may be continued for 4 to 16 hours. Dose response curves may be prepared and an IC 50 value may be calculated for the antibody of interest. In one embodiment, the antibody which blocks ligand activation of an ErbB receptor will have an IC 60 for inhibiting HRG binding to MCF7 cells in this assay of about 50nM or less, more preferably 10nM or less. Where the antibody is an antibody fragment such as a Fab fragment, the IC B0 for inhibiting HRG binding to MCF7 cells in this assay may, for example, be about l OOnM or less, more preferably 50nM or less. Alternatively, or additionally, the ability of the anti-ErbB2 antibody to block ErbB ligand-stimulated tyrosine phosphorylation of an ErbB receptor present in an ErbB hetero-oligomer may be assessed. For example, cells endogenously expressing the ErbB receptors or transfected to expressed them may be incubated with the antibody and then assayed for ErbB ligand-dependent tyrosine phosphorylation activity using an anti-phosphotyrosine monoclonal (which is optionally conjugated with a detectable label). The kinase receptor activation assay described in U.S. Patent No. 5,766,863 is also available for determining ErbB receptor activation and blocking of that activity by an antibody. In one embodiment, one may screen for an antibody which inhibits HRG stimulation of pi 80 tyrosine phosphorylation in MCF7 cells. For example, the MCF7 cells may be plated in 24-well plates and monoclonal antibodies to ErbB2 may be added to each well and incubated for 30 minutes at room temperature; then rHRG 1 177 244 may be added to each well to a final concentration of 0.2 nM, and the incubation may be continued for 8 minutes. Media may be aspirated from each well, and reactions may be stopped by the addition of 100 I of SDS sample buffer (5% SDS, 25 mM DTT, and 25 mM Tris-HCI, pH 6.8). Each sample (25 I) may be electrophoresed on a 4-12% gradient gel (Novex) and then electrophoretically transferred to polyvinylidene difluoride membrane. Antiphosphotyrosine (at 1 g/ml) immunoblots may be developed, and the intensity of the predominant reactive band at M r " 180,000 may be quantified by reflectance densitometry. The antibody selected will preferably significantly inhibit HRG stimulation of p180 tyrosine phosphorylation to about 0-35% of control in this assay. A dose-response curve for inhibition of HRG stimulation of pi 80 tyrosine phosphorylation as determined by reflectance densitometry may be prepared and an IC 5D for the antibody of interest may be calculated. In one embodiment, the antibody which blocks ligand activation of an ErbB receptor will have an IC 50 for inhibiting HRG stimulation of p180 tyrosine phosphorylation in this assay of about 50nM or less, more preferably 10nM or less. Where the antibody is an antibody fragment such as a Fab fragment, the IC 50 for inhibiting HRG stimulation of p180 tyrosine phosphorylation in this assay may, for example, be about l OOnM or less, more preferably 50nM or less. One may also assess the growth inhibitory effects of the antibody on MDA-MB-175 cells, e.g. essentially as described in Schaefer etal. Oncogene 15:1385-1394 (1997). According to this assay, MDA-MB-175 cells may treated with an anti-ErbB2 monoclonal antibody (10 g/mL) for 4 days and stained with crystal violet. Incubation with an anti-ErbB2 antibody may show a growth inhibitory effect on this cell line similar to that displayed by monoclonal antibody 2C4. In a further embodiment, exogenous HRG will not significantly reverse this inhibition. Preferably, the antibody will be able to inhibit cell proliferation of MDA-MB-175 cells to a greater extent than monoclonal antibody 4D5 (and optionally to a greater extent than monoclonal antibody 7F3), both in the presence and absence of exogenous HRG. In one embodiment, the anti-ErbB2 antibody of interest may block heregulin dependent association of ErbB2 with ErbB3 in both MCF7 and SK-BR-3 cells as determined in a co-immunoprecipitation experiment substantially more effectively than monoclonal antibody 4D5, and preferably substantially more effectively than monoclonal antibody 7F3. To screen for antibodies which bind to an epitope on ErbB2 bound by an antibody of interest, a routine cross- blocking assay such as that described in Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, Ed Harlow and David Lane (1988), can be performed. Alternatively, or additionally, epitope mapping can be performed by methods known in the art (see, e.g. Figs. 1 A and IB herein). The results obtained in the cell-based assays described above can then be followed by testing in animal, e.g. murine, models, and human clinical trials. In particular, the inability or limited ability of an antibody to treat ErbB2 overexpressing tumors can be demonstrated in the transgenic mouse model disclosed in the present application as described in the Examples below. C. Anti-ErbB antibodv-maytansinoid conjugates (immunoconjugates) Anti-ErbB antibody-maytansinoid conjugates are prepared by chemically linking an anti-ErbB antibody to a maytansinoid molecule without significantly diminishing the biological activity of either the antibody or the maytansinoid molecule. Maytansinoids are well known in the art and can be synthesized by known techniques or isolated from natural sources. Suitable maytansinoids are disclosed, for example, in U.S. Patent No. 5,208,020 and in the other patents and nonpatent publications referred to hereinabove. Preferred maytansinoids are maytansinol and maytansinol analogues modified in the aromatic ring or at other positions of the maytansinol molecule, such as various maytansinol esters. There are many linking groups known in the art for making antibody-maytansinoid conjugates, including, for example, those disclosed in U.S. Patent No. 5,208,020 or EP Patent 0 425 235 B1, and Chari et al. Cancer Research 52: 127-131 (1992). The linking groups include disufide groups, thioether groups, acid labile groups, photolabile groups, peptidase labile groups, or esterase labile groups, as disclosed in the above-identified patents, disulfide and thioether groups being preferred. Conjugates of the antibody and maytansinoid may be made using a variety of bifunctional protein coupling agents such as N-succiπimidγl-3-(2-pyridyldithio) propionate (SPDP), succinimidγl-4-(N-maleimidomethyl) cyclohexane- 1-carboxylate, iminothiolane (IT), bifunctional derivatives of imidoesters (such as dimethyl adipimidate HCL), active esters (such as disuccinimidyl suberate), aldehydes (such as glutareldehyde), bis-azido compounds (such as bis (p- azidobenzoyl) hexanediamine), bis-diazonium derivatives (such as bis-(p-diazoniumbenzoyl)-ethγlenediamine), diisocyanates (such as tolyene 2,6-diisocγanate), and bis-active fluorine compounds (such as 1 ,5-difluoro-2,4- dinitrobenzene). Particularly preferred coupling agents include N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP) (Carlsson etal, Biochem. J. 173:723-737 [1978]) and N-succinimidyl-4-(2-pyridylthio)peπtanoate (SPP) to provide for a disulfide linkage. The linker may be attached to the maytansinoid molecule at various positions, depending on the type of the link. For example, an ester linkage may be formed at the C-3 position having a hydroxyl group, the C-14 position modified with hyrdoxymethyl, the C-15 position modified with a hydroxyl group, and the C-20 position having a hydroxyl group using conventional coupling techniques. In a preferred embodiment, the linkage is formed at the C-3 position of maytansinol or a maytansinol analogue. D. Pharmaceutical formulations Therapeutic formulations of the antibody-maytansinoid conjugates used in accordance with the present invention are prepared for storage by mixing an antibody having the desired degree of purity with optional pharmaceutically acceptable carriers, excipients or stabilizers (Remington's Pharmaceutical Sciences 16th edition, Osol, A. Ed. (1980)), in the form of lyophilized formulations or aqueous solutions. Acceptable carriers, excipients, or stabilizers are nontoxic to recipients at the dosages and concentrations employed, and include buffers such as phosphate, citrate, and other organic acids; antioxidants including ascorbic acid and methionine; preservatives (such as octadecγldimethγlbenzyl ammonium chloride; hexamethonium chloride; benzalkonium chloride, benzethonium chloride; phenol, butyl or benzyl alcohol; alkyl parabens such as methyl or propyl paraben; catechol; resorcinol; cyclohexanol; 3-pentanol; and m-cresol); low molecular weight (less than about 10 residues) polypeptides; proteins, such as serum albumin, gelatin, or immunoglobulins; hγdrophilic polymers such as polγvinylpγrrolidone; amino acids such as glγcine, glutamine, asparagine, histidine, arginine, or lysine; monosaccharides, disaccharides, and other carbohydrates including glucose, mannose, or dextrins; chelating agents such as EDTA; sugars such as sucrose, mannitol, trehalose or sorbitol; salt-forming counter-ions such as sodium; metal complexes (e.g. Zn protein complexes); and/or non-ionic surfactants such as TWEEN , PLURONICS or polyethylene glycol (PEG). Preferred lyophilized anti- ErbB2 antibody formulations are described in WO 97/04801, expressly incorporated herein by reference. The formulation herein may also contain more than one active compound as necessary for the particular indication being treated, preferably those with complementary activities that do not adversely affect each other. For example, it may be desirable to further provide antibodies or antibody-maytansinoid conjugates which bind to EGFR, ErbB2 (e.g. an antibody which binds a different epitope on ErbB2), ErbB3, ErbB4, or vascular endothelial factor (VEGF) in the one formulation. Alternatively, or additionally, the composition may further comprise a chemotherapeutic agent, cytotoxic agent, cytokine, growth inhibitory agent, anti-hormonal agent, and/or cardioprotectant. Such molecules are suitably present in combination in amounts that are effective for the purpose intended. The active ingredients may also be entrapped in microcapsules prepared, for example, by coacervation techniques or by interfacial polymerization, for example, hydroxymethγlcellulose or gelatin-microcapsules and poly- (methyl ethacylate) microcapsules, respectively, in colloidal drug delivery systems (for example, liposomes, albumin microspheres, microemulsions, nano-particles and nanocapsules) or in macroemulsions. Such techniques are disclosed in Remington's Pharmaceutical Sciences 16th edition, Osol, A. Ed. (1980). Sustained-release preparations may be prepared. Suitable examples of sustained-release preparations include semipermeable matrices of solid hγdrophobic polymers containing the antibody, which matrices are in the form of shaped articles, e.g. films, or microcapsules. Examples of sustained-release matrices include polyesters, hγdrogels (for example, poly(2-hydroxyethyl-methacrylate), or poly(vinylalcohol)), polylactides (U.S. Pat. No. 3,773,919), copolymers of L-glutamic acid and ethyl-L-glutamate, non-degradable ethylene-vinγl acetate, degradable lactic acid- glycolic acid copolymers such as the LUPRON DEPOT (injectable microspheres composed of lactic acid-glycolic acid copolymer and leuprolide acetate), and poly-D-(-)-3-hydroxybutγric acid. The formulations to be used for in vivo administration must be sterile. This is readily accomplished by filtration through sterile filtration membranes. E. Treatment with the anti-ErbB2 antibody-maytansinoid conjugates It is contemplated that, according to the present invention, the anti-ErbB2 antibody-maytansinoid conjugates may be used to treat various diseases or disorders. Exemplary conditions or disorders include benign or malignant tumors; leukemias and lymphoid malignancies; other disorders such as neuronal, glial, astrocytal, hypothalamic, glandular, macrophagal, epithelial, stromal, blastocoelic, inflammatory, angiogenic and immunologic disorders. Generally, the disease or disorder to be treated is cancer. Examples of cancer to be treated herein include, but are not limited to, carcinoma, lymphoma, blastoma, sarcoma, and leukemia or lymphoid malignancies. More particular examples of such cancers include squamous cell cancer (e.g. epithelial squamous cell cancer), lung cancer including small-cell lung cancer, non-small cell lung cancer, adenocarcinoma of the lung and squamous carcinoma of the lung, cancer of the peritoneum, hepatocellular cancer, gastric or stomach cancer including gastrointestinal cancer, pancreatic cancer, glioblastoma, cervical cancer, ovarian cancer, liver cancer, bladder cancer, hepatoma, breast cancer, colon cancer, rectal cancer, colorectal cancer, endometrial or uterine carcinoma, salivary gland carcinoma, kidney or renal cancer, prostate cancer, vulval cancer, thyroid cancer, hepatic carcinoma, anal carcinoma, penile carcinoma, as well as head and neck cancer. The cancer will comprise ErbB-expressing cells, such that an anti-ErbB antibody herein is able to bind to the cancer, and will be typically characterized by overexpression of the ErbB receptor. In a preferred embodiment, the cancer comprises ErbB2-expressing cells, even more preferably, cells which are characterized by overexpression of the ErbB2 receptor. To determine ErbB, e.g. ErbB2 expression in the cancer, various diagnostic/prognostic assays are available. In one embodiment, ErbB2 overexpression may be analyzed by IHC, e.g. using the HERCEPTEST® (Dako). Parrafin embedded tissue sections from a tumor biopsy may be subjected to the IHC assay and accorded a ErbB2 protein staining intensity criteria as follows: Score 0 no staining is observed or membrane staining is observed in less than 10% of tumor cells. Score 1 + a faint/barely perceptible membrane staining is detected in more than 10% of the tumor cells. The cells are only stained in part of their membrane. Score 2+ a weak to moderate complete membrane staining is observed in more than 10% of the tumor cells. Score 3 + a moderate to strong complete membrane staining is observed in more than 10% of the tumor cells. Those tumors with 0 or 1 + scores for ErbB2 overexpression assessment may be characterized as not overexpressing ErbB2, whereas those tumors with 2+ or 3+ scores may be characterized as overexpressing ErbB2. Alternatively, or additionally, FISH assays such as the INFORM (sold by Ventana, Arizona) or PATHVISION (Vysis, Illinois) may be carried out on formalin-fixed, paraffin-embedded tumor tissue to determine the extent (if any) of ErbB2 overexpression in the tumor. In one embodiment, the cancer will be one which expresses (and may overexpress) EGFR. Examples of cancers which may express/overexpress EGFR include squamous cell cancer (e.g. epithelial squamous cell cancer), lung cancer including small-cell lung cancer, non-small cell lung cancer, adenocarcinoma of the lung and squamous carcinoma of the lung, cancer of the peritoneum, hepatocellular cancer, gastric or stomach cancer including gastrointestinal cancer, pancreatic cancer, glioblastoma, cervical cancer, ovarian cancer, liver cancer, bladder cancer, hepatoma, breast cancer, colon cancer, rectal cancer, colorectal cancer, endometrial or uterine carcinoma, salivary gland carcinoma, kidney or renal cancer, prostate cancer, vulval cancer, thyroid cancer, hepatic carcinoma, anal carcinoma, penile carcinoma, as well as head and neck cancer. Preferably, the immunoconjugates of the present invention and/or ErbB, e.g. ErbB2 or EGFR protein to which they are bound are internalized by the cell, resulting in increased therapeutic efficacy of the immunoconjugate in killing the cancer cell to which they bind. In a preferred embodiment, the cytotoxic agent (maytansinoid) targets or interferes with nucleic acid in the cancer cell. The treatment of the present invention targets ErbB overexpressing tumors that do not respond, or respond poorly, to treatment with an unconjugated anti-ErB antibody. Such patients might have received prior treatment with an anti-ErB antibody not conjugated to a maytansinoid moiety, where the prior treatment either did not result in significant improvement, or resulted in transient response. Prior treatment of any particular patient with an unconjugated anti-ErbB antibody is, however, not a prerequisite of identifying patients who are candidates for treatment in accordance with the present invention. An ordinary skilled physician can readily identify patients who are expected to benefit from treatment with the immunoconjugates of the present invention based on publicly available clinical data his or her own experience. Treatment of mammals, and in particular human patients, with or without prior treatment with an (unconjugated) anti-ErbB antibody is specifically within the scope of the present invention. The anti-ErbB antibody-maytansinoid conjugates are administered to a mammal, preferably to a human patient in accord with known methods, such as intravenous administration, e.g., as a bolus or by continuous infusion over a period of time, by intramuscular, intraperitoneal, intracerobrospinal, subcutaneous, intra-articular, intrasynovial, intrathecal, oral, topical, or inhalation routes. Intravenous or subcutaneous administration of the antibody is preferred. Other therapeutic regimens may be combined with the administration of the anti-ErbB antibody-maytansinoid conjugates. The combined administration includes coadministration, using separate formulations or a single pharmaceutical formulation, and consecutive administration in either order, wherein preferably there is a time period while both (or all) active agents simultaneously exert their biological activities. In one preferred embodiment, the patient is treated with two or more different anti-ErbB antibodies, at least one of which is in the form of a maytansinoid conjugate. For example, the patient may be treated with a first anti- ErbB2 antibody-maytansinoid conjugate in which the antibody is growth inhibitory (e.g. HERCEPTIN®), and a second anti-ErbB2 antibody or antibody-immunoconjugate, e.g. an antibody-maytansinoid conjugate which blocks ligand activation of an ErbB receptor (e.g. 2C4 or a humanized and/or affinity matured variant thereof) or induces apoptosis of an ErbB2-overexpressing cell (e.g. 7C2, 7F3 or humanized variants thereof). In another embodiment, the treatment involves the administration of antibodies that specifically bind two or more different ErbB receptors, such as, for example, ErbB2 and EGFR receptors, where at least one of the anti-ErbB antibodies is administered as a maytansinoid conjugate. Preferably such combined therapy results in a synergistic therapeutic effect. It may also be desirable to combine administration of the anti-ErbB antibody-maytansinoid conjugates, with administration of an antibody directed against another tumor-associated antigen, which is not member of the ErbB family of receptors. The other antibody in this case may, for example, bind to vascular endothelial growth factor (VEGF), and may be in the form of a maytansinoid conjugate, or another immunoconjugate. In one embodiment, the treatment of the present invention involves the combined administration of an anti- ErbB2 antibody-maytansinoid conjugate (or conjugates) and one or more chemotherapeutic agents or growth inhibitory agents, including coadministration of cocktails of different chemotherapeutic agents. Preferred chemotherapeutic agents include taxanes (such as paclitaxel and doxetaxel) and/or anthracycliπe antibiotics. Preparation and dosing schedules for such chemotherapeutic agents may be used according to manufacturers' instructions or as determined empirically by the skilled practitioner. Preparation and dosing schedules for such chemotherapy are also described in Chemotherapy Service Ed., M.C. Perry, Williams & Wilkins, Baltimore, MD (1992). The antibody-maytansinoid conjugates may be combined with an anti-hormonal compound; e.g., an anti- estrogen compound such as tamoxifen; an anti-progesterone such as onapristone (see, EP 616 812); or an anti- androgen such as flutamide, in dosages known for such molecules. Where the cancer to be treated is hormone independent cancer, the patient may previously have been subjected to anti-hormonal therapy and, after the cancer becomes hormone independent, the anti-ErbB2 antibody (and optionally other agents as described herein) may be administered to the patient. Sometimes, it may be beneficial to also coadminister a cardioprotectant (to prevent or reduce myocardial dysfunction associated with the therapy) or one or more cytokines to the patient. In addition to the above therapeutic regimes, the patient may be subjected to surgical removal of cancer cells and/or radiation therapy. Suitable dosages for any of the above coadministered agents are those presently used and may be lowered due to the combined action (synergy) of the agent and anti-ErbB2 antibody. For the prevention or treatment of disease, the appropriate dosage of antibody-maytansinoid conjugates will depend on the type of disease to be treated, as defined above, the severity and course of the disease, whether the antibody is administered for preventive or therapeutic purposes, previous therapy, the patient's clinical history and response to the antibody, and the discretion of the attending physician. The antibody-maytansinoid conjugate is suitably administered to the patient at one time or over a series of treatments. Depending on the type and severity of the disease, about 1 g/kg to 15 g/kg (e.g. 0.1-20mg/kg) of antibody-maytansinoid conjugate is an initial candidate dosage for administration to the patient, whether, for example, by one or more separate administrations, or by continuous infusion. A typical daily dosage might range from about 1 g/kg to 100 mg/kg or more, depending on the factors mentioned above. For repeated administrations over several days or longer, depending on the condition, the treatment is sustained until a desired suppression of disease symptoms occurs. A preferred dosing regimen comprises administering an initial loading dose of about 4 mg/kg, followed by a weekly maintenance dose of about 2 mg/kg of the anti-ErbB2 antibody-maytansinoid conjugate. However, other dosage regimens may be useful. The progress of this therapy is easily monitored by conventional techniques and assays. F. Articles of Manufacture In another embodiment of the invention, an article of manufacture containing materials useful for the treatment of the disorders described above is provided. The article of manufacture comprises a container and a label or package insert on or associated with the container. Suitable containers include, for example, bottles, vials, syringes, etc. The containers may be formed from a variety of materials such as glass or plastic. The container holds a composition which is effective for treating the condition and may have a sterile access port (for example the container may be an intravenous solution bag or a vial having a stopper pierceable by a hypodermic injection needle). At least one active agent in the composition is an anti-ErbB2 antibody-maytansinoid conjugate. The label or package insert indicates that the composition is used for treating the condition of choice, such as cancer. In one embodiment, the label or package inserts indicates that the composition comprising the antibody which binds ErbB2 can be used to treat cancer which expresses an ErbB receptor selected from the group consisting of epidermal growth factor receptor (EGFR), ErbB2, ErbB3 and ErbB4, preferably EGFR. In addition, the label or package insert may indicate that the patient to be treated is one having cancer characterized by excessive activation of an ErbB receptor selected from EGFR, ErbB2, ErbB3 or ErbB4. For example, the cancer may be one which overexpresses one of these receptors and/or which overexpresses an ErbB ligand (such as TGF- ). The label or package insert may also indicate that the composition can be used to treat cancer, wherein the cancer is not characterized by overexpression of the ErbB2 receptor. For example, whereas the present package insert for HERCEPTIN® indicates that the antibody is used to treat patients with metastatic breast cancer whose tumors overexpress the ErbB2 protein, the package insert herein may indicate that the antibody or composition is used to treat cancer that does not respond, or respond poorly, to treatment with HERCEPTIN®. In other embodiments, the package insert may indicate that the antibody-maytansinoid conjugate or composition can be used also to treat hormone independent cancer, prostate cancer, colon cancer or colorectal cancer. Moreover, the article of manufacture may comprise (a) a first container with a composition contained therein, wherein the composition comprises a maytansinoid conjugate of a first antibody which binds ErbB2 and inhibits growth of cancer cells which overexpress ErbB2; and (b) a second container with a composition contained therein, wherein the composition comprises a second antibody which binds ErbB2 and blocks ligand activation of an ErbB receptor, or a conjugate of this second antibody with a maytansinoid. The article of manufacture in this embodiment of the invention may further comprises a package insert indicating that the first and second compositions can be used to treat cancer. Alternatively, or additionally, the article of manufacture may further comprise a second (or third) container comprising a pharmaceutically-acceptable buffer, such as bacteriostatic water for injection (BWFI), phosphate-buffered saline, Ringer's solution and dextrose solution. It may further include other materials desirable from a commercial and user standpoint, including other buffers, diluents, filters, needles, and syringes. Further details of the invention are illustrated in the following non-limiting examples. Example 1 Production, Characterization and Humanization of Anti ErbB2 Monoclonal Antibody 4D5 The murine monoclonal antibody 4D5 which specifically binds the extracellular domain of ErbB2 was produced as described in Fendly et al. Cancer Research 50:1550-1558 (1990). Briefly, NIH 3T3/HER2-3 400 cells (expressing approximately 1 x 10 5 ErbB2 molecules/cell) produced as described in Hudziak et al Proc. Natl. Acad. Sci. (USA) 84:7158-7163 (1987) were harvested with phosphate buffered saline (PBS) containing 25mM EDTA and used to immunize BALB/c mice. The mice were given injections i.p. of 10 7 cells in 0.5ml PBS on weeks 0, 2, 5 and 7. The mice with antisera that immunoprecipitated 32 P-labeled ErbB2 were given i.p. injections of a wheat germ agglutinin- Sepharose (WGA) purified ErbB2 membrane extract on weeks 9 and 13. This was followed by an i.v. injection of 0.1 ml of the ErbB2 preparation and the splenocytes were fused with mouse myeloma line X63-Ag8.653. Hybridoma superπatants were screened for ErbB2-binding by ELISA and radioimmunoprecipitation. Epitope mapping and characterization The ErbB2 epitope bound by monoclonal antibody 4D5 was determined by competitive binding analysis (Fendly etal. Cancer Research 50:1550 -1558 (1990)). Cross-blocking studies were done by direct fluorescence on intact cells using the PANDEX™ Screen Machine to quantitate fluorescence. The monoclonal antibody was conjugated with fluorescein isothiocyanate (FITC), using established procedures (Wofsy et al Selected Methods in Cellular Immunology, p. 287, Mishel and Schiigi (eds.) San Francisco: W.J. Freeman Co. (1980)). Confluent monolayers of NIH 3T3/HER2-3 400 cells were trypsinized, washed once, and resuspended at 1.75 x I O 6 cell/ml in cold PBS containing 0.5% bovine serum albumin (BSA) and 0.1 % NaN 3 . A final concentration of 1 % latex particles (IDC, Portland, OR) was added to reduce clogging of the PANDEX™ plate membranes. Cells in suspension, 20 I, and 20 I of purified monoclonal antibodies (100 g/ml to 0.1 g/ml) were added to the PANDEX™ plate wells and incubated on ice for 30 minutes. A predetermined dilution of the FITC-labeled monoclonal antibody in 20 I was added to each well, incubated for 30 minutes, washed, and the fluorescence was quantitated by the PANDEX™. Monoclonal antibodies were considered to share an epitope if each blocked binding of the other by 50% or greater in comparison to an irrelevant monoclonal antibody control. In this experiment, monoclonal antibody 4D5 was assigned epitope I (amino acid residues from about 529 to about 625, inclusive within the ErbB2 extracellular domain (see SEQ ID NO: 3). The growth inhibitory characteristics of monoclonal antibody 4D5 were evaluated using the breast tumor cell line, SK-BR-3 (see Hudziak et al Mo/ec. Cel Biol. 9(31:1165-1172 (1989)). Briefly, SK-BR-3 cells were detached by using 0.25% (vol/vol) trypsin and suspended in complete medium at a density of 4 x 10 5 cells per ml. Aliquots of 100 I (4 x 10 4 cells) were plated into 96-well microdilution plates, the cells were allowed to adhere, and 100 I of media alone or media containing monoclonal antibody (final concentration 5 g/ml) was then added. After 72 hours, plates were washed twice with PBS (pH 7.5), stained with crystal violet (0.5% in methanol), and analyzed for relative cell proliferation as described in Sugarman et al. Science 230:943-945 (1985). Monoclonal antibody 4D5 inhibited SK- BR-3 relative cell proliferation by about 56%. Monoclonal antibody 4D5 was also evaluated for its ability to inhibit HRG-stimulated tyrosine phosphorylation of proteins in the M r 180,000 range from whole-cell Iγsates of MCF7 cells (Lewis et at. Cancer Research 56:1457-1465 (1996)). MCF7 cells are reported to express all known ErbB receptors, but at relatively low levels. Since ErbB2, ErbB3, and ErbB4 have nearly identical molecular sizes, it is not possible to discern which protein is becoming tyrosine phosphorylated when whole-cell lysates are evaluated by Western blot analysis. However, these cells are ideal for HRG tyrosine phosphorylation assays because under the assay conditions used, in the absence of exogenously added HRG, they exhibit low to undetectable levels of tyrosine phosphorylation proteins in the M, 180,000 range. MCF7 cells were plated in 24-well plates and monoclonal antibodies to ErbB2 were added to each well and incubated for 30 minutes at room temperature; then rHRG 1 177244 was added to each well to a final concentration of 0.2 nM, and the incubation was continued for 8 minutes. Media was carefully aspirated from each well, and reactions were stopped by the addition of 100 I of SDS sample buffer (5% SDS, 25 mM DTT, and 25 mM Tris-HCI, pH 6.8). Each sample (25 I) was electrophoresed on a 4-12% gradient gel (Novex) and then electrophoreticallγ transferred to polyvinylidene difluoride membrane. Antiphosphotyrosine (4G10, from UBI, used at 1 g/ml) immunoblots were developed, and the intensity of the predominant reactive band at M, 180,000 was quantified by reflectance densitometry, as described previously (Holmes eta Science 256:1205-1210 (1992); Sliwkowski etal. J. Biol Chem. 269:14661-14665 (1994)) Monoclonal antibody 4D5 significantly inhibited the generation of a HRG-induced tyrosine phosphorylation signal at M, 180,000. In the absence of HRG, but was unable to stimulate tyrosine phosphorylation of proteins in the M, 180,000 range. Also, this antibody does not cross-react with EGFR (Fendly et a Cancer Research 50:1550-1558 (1990)), ErbB3, or ErbB4. Monoclonal antibody 4D5 was able to block HRG stimulation of tyrosine phosphorylation by 50%. The growth inhibitory effect of monoclonal antibody 4D5 on MDA-MB-175 and SK-BR-3 cells in the presence or absence of exogenous rHRG 1 was assessed /Schaefer et al. Oncogene 15:1385-1394 (1997)). ErbB2 levels in MDA-MB-175 cells are 4-6 times higher than the level found in normal breast epithelial cells and the ErbB2-ErbB4 receptor is constitutively tyrosine phosphorylated in MDA-MB-175 cells. Monoclonal antibody 4D5 was able to inhibit cell proliferation of MDA-MB-175 cells, both in the presence and absence of exogenous HRG. Inhibition of cell proliferation by 4D5 is dependent on the ErbB2 expression level (Lewis et at. Cancer Immuno Immunother. 37:255- 263 (1993)). A maximum inhibition of 66% in SK-BR-3 cells could be detected. However this effect could be overcome by exogenous HRG. Humaπization The murine monoclonal antibody 4D5 was humanized, using a novel "gene conversion mutagenesis" strategy, as described in U.S. Patent No. 5,821,337, the entire disclosure of which is hereby expressly incorporated by reference. The humanized monoclonal antibody 4D5 used in the following experiments is designated huMAb4D5-8. This antibody is of IgGI isotype. Example 2 HERCEPTIN ® -DM1 conjugates 1. Purification of HERCEPTIN ® HERCEPTIN ® ( huMAb4D5-8, rhuMAb HER2, U.S. Patent No. 5,821,337) (1 vial containing 440 mg antibody) was dissolved in 50 mL MES buffer (25 mM MES, 50 mM NaCI, pH 5.6). The sample was loaded on a cation exchange column (Sepharose S, 15 cm x 1.7 cm) that had been equilibrated in the same buffer. The column was then washed with the same buffer (5 column volumes). HERCEPTIN ® was eluted by raising the NaCI concentration of the buffer to 200 mM. Fractions containing the antibody were pooled, diluted to 10 mg/mL, and dialyzed into a buffer containing 50 mm potassium phosphate, 50 mM NaCI, 2 mM EDTA, pH 6.5. 2. Modification of HERCEPTIN ® with SPP The purified HERCEPTIN ® antibody was modified with N-succinimidyl-4-(2-pyridylthio)pentanoate (SPP) to introduce dithiopyridyl groups. The antibody (376.0 mg, 8 mg/mL) in 44.7 mL of 50 mM potassium phosphate buffer (pH 6.5) containing NaCI (50 mM) and EDTA (1 mM) was treated with SPP (5.3 molar equivalents in 2.3 L ethanol). After incubation for 90 minutes under argon at ambient temperature, the reaction mixture was gel filtered through a Sephadex G25 column equilibrated with 35 mM sodium citrate, 154 mM NaCI, 2 mM EDTA. Antibody containing fractions were pooled and assayed. The degree of modification of the antibody was determined as described above. Recovery of the modified antibody (HERCEPTIN ® -SPP-Py) was 337 mg (89.7%) with 4.5 releasable 2-thiopyridine groups linked per antibody. 3. Conjugation of HERCEPTIN ® -SPP Pv with DM1 The modified antibody (337.0 mg, 9.5 μ ols of releasable 2-thiopyridine groups) was diluted with the above 35 mM sodium citrate buffer, pH 6.5, to a final concentration of 2.5 mg/mL. DM1 (the structure of which is shown in Figure 1) (1.7 equivalents, 16.1 μmols) in 3.0 mM dimethylacetamide (DMA, 3% v/v in the final reaction mixture) was then added to the antibody solution. The reaction proceeded at ambient temperature under argon for 20 hours. The structure of HERCEPTIN ® -DM1 conjugates is illustrated in Figure 2. The reaction was loaded on a Sephacryl S300 gel filtration column (5.0 cm x 90.0 cm, 1.77 L) equilibrated with 35 mM sodium citrate, 154 mM NaCI, pH 6.5. The flow rate was 5.0 mL/miπ and 65 fractions (20.0 mL each) were collected. A major peak centered around fraction No. 47 (Figure 3). The major peak comprises monomeric HERCEPTIN ® -DM1. Fractions 44-51 were pooled and assayed. The number of DM1 drug molecules linked per antibody molecule was determined by measuring the absorbance at 252 nm and 280 nm, and found to be 3.7 drug molecules per antibody molecule. Example 3 Transgenic Animals In order to improve the clinical activity of HERCEPTIN ® , a transgenic HER2 mouse model was developed in which novel HER2-directed therapies could be tested preclinicallγ. Tumors arise readily in transgenic mice that express a mutationally activated form of neu, the rat homolog of HER2, but the HER2 that is overexpressed in breast cancers is not mutated and tumor formation is much less robust in transgenic mice that overexpress nonmutated HER2 (Webster etal, Semin. Cancer Biol. 5: 69-76 [1994]). To improve tumor formation with nonmutated HER2, a strategy was used to further enhance overexpression of nonmutated HER2 in a transgenic mouse. Any promoter that promotes expression of HER2 in epithelial cells in the mouse mammary gland can be used in the disclosed constructs. Many of the milk protein genes are transcribed by promoter/enhancer elements that are specifically active in mammary glands. Milk protein genes include those genes encoding caseins (α-S, and β), β- lactoglobulin, α-lactalbumin, and whey acidic protein. The ovine β-lactoglobulin promoter is well characterized and widely used in the art (Whitelaw et al, Biochem J. 286: 31-39, [1992]). However, similar fragments of promoter DNA from other species are also suitable. A preferred promoter is the promoter derived from the Long Terminal Repeat (LTR) of the Mouse Mammary Tumor Virus (MMTV). A HER2 transgene construct of the present invention was generated using the MMTV LTR promoter. To improve tumor formation with nonmutated HER2, we have made transgenic mice using a HER2 cDNA plasmid in which an upstream ATG was deleted in order to prevent initiation of translation at such upstream ATG codons, which would otherwise reduce the frequency of translation initiation from the downstream authentic initiation codon of HER2 (for example, see Child et al, J. Biol. Chem. 274: 24335-24341 [1999]). Additionally, a chimeric intron was added to the 5' end, which should also enhance the level of expression as reported earlier (Neuberger and Williams, Nucleic Acids Res. 16: 6713 [1988]; Buchman and Berg, Mol. Cell. Biol. 8: 4395 [1988]; Brinster et al, Proc. Natl. Acad. Sci. USA 85: 836 [1988]). The chimeric intron was derived from a Promega vector, pCI-neo mammalian expression vector (bp 890-1022). The cDNA 3'-end is flanked by human growth hormone exons 4 and 5, and polyadenylation sequences. Moreover, FVB mice were used because this strain is more susceptible to tumor development. The promoter from MMTV-LTR was used to ensure tissue-specific HER2 expression in the mammary gland. Animals were fed the AIN 76A diet in order to increase susceptibility to tumor formation (Rao et al, Breast CancerRes. and Treatment ^: 149-158 [1997]). The nucleotides sequence of this transgene plasmid construct (SEQ ID NO: 1 ) is shown in Figure 4. Animals suitable for transgenic experiments can be obtained from standard commercial sources such as Taconic (Germantown, N.Y.). Many strains are suitable, but FVB female mice are preferred because of their higher susceptibility to tumor formation. FVB males were used for mating and vasectomized CD.1 studs were used to stimulate pseudopregnancγ. Vasectomized mice can be obtained from any commercial supplier. Founders were bred with either FVB mice or with 129/BL6 x FVB p53 heterozygous mice. The mice with heterozygosity at p53 allele were used to potentially increase tumor formation. However, this has proven unecessary. Therefore, some F1 tumors are of mixed strain. Founder tumors are FVB only. Six founders were obtained with some developing tumors without having litters. Example 4 HER2 transgenic mouse as a tumor model to evaluate HER2-directed therapies Mammary gland biopsies of one founder transgenic mouse made as described in Example 3, showed 3 + expression of HER2, as determined by immunohistochemical staining, at about 2 months of age. The amount of HER2 extracellular domain (ECD) shed into serum was measured and found to be about 1.2 ng/ml (Huang et al., supra). This mouse subsequently developed a mammary tumor at 5 months of age, after bearing 4 litters. The tumor was surgically resected under aseptic conditions and minced into small pieces, 2 mm 3 , which were then transplanted into the mammary fat pad of wild-type FVB female mice. Tumors developed in 22 of 31 recipient mice, with a latency of 5 weeks. With subsequent passage, tumors developed with shorter latency and grew more rapidly, and tumor incidence increased to > 95% of recipients. HER2 expression, as determined by immunohistochemical staining, was 3 + but heterogeneous in the primary tumor, but became uniformly 3+ after the first passage. Treatment of tumor-bearing mice with HERCEPTIN ® or 4D5, the murine antibody from which humanized HERCEPTIN ® was derived, had only a modest effect on the growth of the transplanted tumors (Figure 5). HER2 expression was 3+ in tumors that grew during HERCEPTIN ® or 4D5 therapy, indicating that there was no selection of HER2-negative tumors. Moreover, cy3-HERCEPTIN ® was detected decorating tumor cells after injection into tumor- bearing mice, indicating that the lack of efficacy was not due to failure of the antibody to access the tumor. Based on the persistent expression of HER2 and the failure of this tumor model to respond to HERCEPTIN ® , a novel approach was tested, using HERCEPTIN ® conjugated to maytansinoid DM1 as described in Example 3. Figure 5 shows that the HERCEPTIN ® -DM1 conjugate has dramatic anti-tumor activity in this model. RITUXAN ® , an unrelated anti-CD20 monoclonal antibody, was used as a negative control for these studies. There was little response to HERCEPTIN ® compared to the control antibody, RITUXAN* , but there was striking anti-tumor activity of the maytansinoid conjugate of HERCEPTIN ® . As shown in Figure 5, all of the mice treated with HERCEPTIN ® - maytansinoid showed striking shrinkage of their tumors, though none of the tumors disappeared. After approximately 4 weeks, tumors began to regrow. Five animals were sacrificed at this time. Their tumors were found to express HER2 at 3+ levels. Thus, there was no selection for HER2-negative tumors. Based on this observation, the remaining 3 mice were treated with HERCEPTIN ® -maytansinoid for 5 consecutive days. The tumors again regressed in response to the treatment. All references cited throught the specification, and the references cited therein, are hereby expressly incorporated by reference. Deposit of Biological Material The following hybridoma cell lines have been deposited with the American Type Culture Collection, 10801 University Boulevard, Manassas, VA 20110-2209, USA (ATCC): Antibody Designation ATCC No. Deposit Date 7C2 ATCC HB-12215 October 17, 1996 7F3 ATCC HB-12216 October 17, 1996 4D5 ATCC CRL 10463 May 24, 1990 2C4 ATCC HB-12697 April 8, 1999 This deposit was made under the provisions of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purpose of Patent Procedure and the Regulations thereunder (Budapest Treaty). This assures maintenance of viable cultures for 30 years from the date of the deposit. The organisms will be made available by ATCC under the terms of the Budapest Treaty, and subject to an agreement between Genentech, Inc. and ATCC, which assures permanent and unrestricted availability of the progeny of the cultures to the public upon issuance of the pertinent U.S. patent or upon laying open to the public of any U.S. or foreign patent application, whichever comes first, and assures availability of the progeny to one determined by the U.S. Commissioner of Patents and Trademarks to be entitled thereto according to 35 USC §122 and the Commissioner's rules pursuant thereto (including 37 CFR §1.12 with particular reference to 886 OG 638). In respect of those designations in which a European patent is sought, a sample of the deposited microorganism will be made available until the publication of the mention of the grant of the European patent or until the date on which the application has been refused or withdrawn or is deemed to be withdrawn, only by the issue of such a sample to an expert nominated by the person requesting the sample. (Rule 28(4) EPC) The assignee of the present application has agreed that if the cultures on deposit should die or be lost or destroyed when cultivated under suitable conditions, they will be promptly replaced on notification with a viable specimen of the same culture. Availability of the deposited strain is not to be construed as a license to practice the invention in contravention of the rights granted under the authority of any government in accordance with its patent laws. The foregoing written specification is considered to be sufficient to enable one skilled in the art to practice the invention. The present invention is not to be limited in scope by the constructs deposited, since the deposited embodiments are intended to illustrate only certain aspects of the invention and any constructs that are functionally equivalent are within the scope of this invention. The deposit of material herein does not constitute an admission that the written description herein contained is inadequate to enable the practice of any aspect of the invention, including the best mode thereof, nor is it to be construed as limiting the scope of the claims to the specific illustrations that they represent. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and fall within the scope of the appended claims. It is understood that the application of the teachings of the present invention to a specific problem or situation will be within the capabilities of one having ordinary skill in the art in light of the teachings contained herein. Examples of the products of the present invention and representative processes for their isolation, use, and manufacture appear below, but should not be construed to limit the invention.

Claims: What is claimed is: 1. A method for the treatment of a tumor in a mammal, wherein the tumor is characterized by the overexpression of an ErbB receptor and does not respond, or responds poorly, to treatment with an anti-ErbB antibody, comprising administering to the mammal a therapeutically effective amount of a conjugate of the anti-ErbB antibody with a maytansinoid. 2. The method of claim 1 wherein the mammal is human. 3. The method of claim 2 wherein the ErbB receptor is selected from the group consisting of ErbB1 (EGFR), ErbB2 (HER2), ErbB3 (HER3) and ErbB4 (HER4). The method of claim 3 wherein the anti-ErbB antibody is a growth inhibitory antibody. The method of claim 3 wherein the anti-ErbB antibody induces cell death. The method of claim 3 wherein the anti-ErbB antibody induces apoptosis. The method of claim 3 wherein the antibody is an anti-ErbB2 antibody. 8. The method of claim 7 wherein the tumor is cancer. 9. The method of claim 8 wherein the cancer is selected from the group consisting of breast, ovarian, stomach, endometrial, salivary gland, lung, kidney, colon, colorectal, thyroid, pancreatic, prostate and bladder cancer. 10. The method of claim 9 wherein the cancer is breast cancer. 1 1. The method of claim 10 wherein the breast cancer overexpresses ErbB2 at a 2+ level or more. 12. The method of claim 1 1 wherein the breast cancer overexpresses ErbB2 at a 3+ level. 13. The method of claim 12 wherein the antibody has a biological characteristic of a 4D5 monoclonal antibody. 14. The method of claim 13 wherein the antibody binds essentially the same epitope as a 4D5 monoclonal antibody. 15. The method of claim 13 wherein the antibody is the monoclonal antibody 4D5 (ATCC CRL 10463). 16. The method of claim 13 wherein the antibody is humanized. 17. The method of claim 16 wherein the antibody is selected from the group consisting of humanized antibodies huMAb4D5-1, huMAb4D5-2, huMAb4D5-3, huMAb4D5-4, huMAb4D5-5, huMAb4D5-6, huMAb4D5-7 and huMAb4D5-8 (HERCEPTIN ® ). 18. The method of claim 17 wherein the antibody is humanized antibody huMAb4D5-8 (HERCEPTIN ® ). 19. The method of claim 3 wherein the antibody is an antibody fragment. 20. The method of claim 19 wherein the antibody fragment is a Fab fragment. 21. The method of claim 3 wherein the maytansinoid is maytansine. 22. The method of claim 3 wherein the maytansinoid is maytansinol. 23. The method of claim 3 wherein the maytansinoid is a maytansinol ester. 24. The method of claim 23 wherein the maytansinoid is a C-3 ester of maytansinol. 25. The method of claim 24 wherein the maytansioid is DM1 shown in Figure 1. 26. The method of claim 3 wherein the antibody and maytansinoid are conjugated by a bispecific chemical linker. 27. The method of claim 26 wherein said chemical linker is N-succinimidyl-4-(2-pyridγlthio)propanoate (SPDP) or N-succinimidyl-4-(2-pyridylthio)pentanoate (SPP). 28. The method of claim 3 wherein the antibody and maytansinoid are conjugated by a linking group selected from the group consisting of a disulfide, thioether, acid labile, photolabile, peptidase labile, and esterase labile group. 29. The method of claim 28 wherein the linking group is a disulfide or a thioether group. 30. The method of claim 29 wherein the linking group is a disulfide group. 31. The method of claim 3 wherein the conjugate comprises 1 to about 10 maytansinoid molecules per antibody molecule. 32. The method of claim 31 wherein the conjugate comprises from about 3 to about 5 maytansinoid molecules per antibody molecule. 33. The method of claim 7 further comprising the administration of a second antibody which binds ErbB2 and blocks ligand activation of an ErbB receptor. 34. The method of claim 33 wherein the second antibody comprises monoclonal antibody 2C4 or humanized 2C4. 35. The method of claim 33 wherein said second antibody is conjugated with a cytotoxic agent. 36. The method of claim 35 wherein the cytotoxic agent is a maytansinoid. 37. An article of manufacture comprising a container and a composition contained therein, wherein the composition comprises an anti-ErbB antibody-maytansinoid conjugate, and further comprising a package insert or label indicating that the composition can be used to treat cancer characterized by the overexpression of an ErbB receptor. 38. The article of manufacture of claim 37 wherein said package insert of label indicates that the composition can be used to treat cancer characterized by the overexpression of an ErbB2 receptor. 39. The article of manufacture of claim 38 wherein the cancer is breast cancer. 40. The article of manufacture of claim 38 wherein the cancer is characterized by the overexpression of an ErbB2 receptor at a 2+ level or above. 41. The article of manufacture of claim 40 wherein the cancer is characterized by the overexpression of an ErbB2 receptor at a 3+ level.

Inventors: ERICKSON SHARON, SCHWALL RALPH, ERICKSON, SHARON, SCHWALL, RALPH
Applicants: ERICKSON SHARON, GENENTECH INC, SCHWALL RALPH, ERICKSON, SHARON, GENENTECH, INC., SCHWALL, RALPH
Application year: 2000
Main classification: A61K47
IPC codes: A01K67, A61K39, A61K47, A61P1, A61P5, A61P11, A61P13, A61P15, A61P35, C12N15