COMPOUNDS THAT INTERFERE WITH DNA REPLICATION IN RAPIDLY PROLIFERATING CELLS FOR USE IN CANCER THERAPY AND METHODS FOR SCREENING FOR SUCH COMPOUNDS

Abstract: Compounds which interfere with the function of the cdc6 protein to form or maintain pre-replication complexes in an animal cell without impairing the cell's ability to activate cyclin dependent kinases that promote S phase and/or M phase for use in cancer therapy due to their ability to kill tumour cells by inhibiting DNA replication. Method for screening for such compounds. There is now considerable evidence that chromosome duplication is triggered by the activation in late G1 of particular cyclin dependent kinases, known as S phase promoting Cdks. It was an object of the present invention to further investigate the mechanisms involved in DNA replication and, utilizing the obtained results, as opposed to conventional chemotherapy, does not damage DNA per se but interferes with DNA replication.

Description: COMPOUNDS THAT INTERFERE WITH DNA REPLICATION IN RAPIDLY PROLIFERATING CELLS FOR USE IN CANCER THERAPY AND METHODS FOR SCREENING FOR SUCH COMPOUNDS The present invention relates in general to the field of cancer therapy. There is now considerable evidence that chromosome duplication is triggered by the activation in late GI of particular cyclin dependent kinases, known as S phase promoting Cdks. It was an object of the present invention to further investigate the mechanisms involved in DNA replication and, utilizing the obtained results, to provide a novel concept for cancer therapy which, as opposed to conventional chemotherapy, does not damage DNA per se but interferes with DNA replication. In the budding yeast Saccharomyces cerevisiae, entry into S phase depends on Cdks formed by the association of one of six unstable B-type cyclins, Clbl-6, with a single kinase subunit, Cdkl/Cdc28 (Schwob et al . , 1994) . Cyclin B/Cdkl kinases are also required for the assembly of a mitotic spindle and for the onset of anaphase in yeast (Surana et al . , 1991; Fitch et al . , 1992; Schwob and Nasmyth, 1993) . All six cyclin B/Cdkl kinases are inactivated as cells complete anaphase (Amon et al. , 1994; Irniger et al . , 1995) . Their re-activation during the subsequent GI period involves periodic transcription of cyclin genes (Koch and Nasmyth, 1994) , cessation of cyclin B proteolysis (Amon et al . , 1994) , and, most important, destruction of p40^C i which otherwise inhibits cyclin B/Cdkl kinases through the formation of ternary p40 SIC1 /cyclinB/Cdkl complexes (Mendenhall, 1993; Schwob et al . , 1994) . Inactivation of all six B-type cyclins or mutations in genes like CDC4 and CDC34 , which are needed for p40^C-- proteolysis, cause cells to arrest in GI, whereas inactivation of the SIC1 gene modestly advances S phase (Schwob et al . , 1994) . The equivalent S phase promoting Cdks in metazoa appear to be complexes between an S phase specific kinase subunit Cdk2 and a similar set of cyclins, known as E-type cyclins. It is thought that Clb5/ and Clbβ/Cdkl kinases normally trigger S phase in yeast (Epstein and Cross, 1992; Schwob and Nasmyth, 1993) ; these are the first B-type cyclins to appear due to transcriptional controls that cause CLB5 and CLBfS mRNAs to accumulate in late GI . CLB3 and CLB4 are transcribed in S and G2 phases, while CLB1 and CLB2 are transcribed in G2 and M phases (Koch and Nasmyth, 1994) . Deletion of both CLB5 and CLB6 is not lethal but causes S phase to be delayed by about 30 minutes (Schwob and Nasmyth, 1993) , i.e. until the later Clbs have had time to accumulate. It is now clear that one or another of the six S phase promoting cyclinB/Cdkl kinases are active from the time of p40 SIC1 destruction in late GI to the activation of cyclin B proteolysis during anaphase and yet origins ' fire once and only once during this wide window of the yeast cell cycle. It is possible that S phase promoting Cdks trigger chromosome duplication in GI but not in G2 cells because the appropriate substrates for S phase promoting Cdks might only exist in GI cells? Proteins that bind to origins of DNA replication might be the key substrates for Cdks and their availability might vary during the cell cycle. A large multisubunit complex called the Origin Recognition Complex (ORC) binds to and is required for the activity of yeast origins (Bell and Stillman, 1992; Micklem et al . , 1993; Liang et al . , 1995; Fox et al . , 1995) . ORC is bound to origins at all stages of the cell cycle (Diffley and Cocker, 1992) , but the DNasel sensitivity of the neighbouring chromatin changes during the cycle (Diffley et al., 1994) . Sequences adjacent to ORC are protected from DNasel during GI but not during G2 or M phase. The pre-replicative "protected" state of origins arises at the end of mitosis, following the inactivation of Clb/Cdkl kinases and lasts until the beginning or middle of S phase (Diffley et al . , 1994) . The formation and maintainance of the pre-replicative state of origins depends on the synthesis of an unstable protein encoded by the CDC6 gene (Cocker et al . , 1996) , which is essential for the initiation of DNA replication (Hartwell, 1976; Bueno and Russell, 1992; Piatti et al . , 1995) . Defects in the replication of minichromosomes in cdcβ mutants can be alleviated by adding extra origins, suggesting that Cdc6p (CdC6 protein) acts at origins (Hogan and Koshland, 1992) . Cdc6 transcription occurs in two bursts during the cycle: at the end of mitosis, coinciding with the switch of origins from post-replicative to pre- replicative state, and in late GI (Zwerschke et al . , 1994; Piatti et al . , 1995) . Cells that exit from mitosis without de novo Cdc6 protein synthesis fail both to establish the pre-replicative state at their origins (Cocker et al. , 1996) and to initiate DNA replication, despite activating S phase promoting Cdks on schedule; such cells proceed with the alignment of unreplicated chromosomes on mitotic spindles and finally undergo a "reductional" anaphase (Piatti et al. , 1995) . These data suggest that Cdc6p synthesis is required for the assembly of protein complexes adjacent to ORC (pre-replicative complexes or pre-RCs) . The formation of such pre-RCs at the end of mitosis or in late GI (when there is a second burst of Cdc6 synthesis) might be the first of two steps needed for the initiation of DNA replication; the second step being the activation of cyclin B/Cdkl kinases. According to this notion, the presence of pre-RCs in GI but not in G2 cells could explain why S phase promoting Cdks trigger only the former to enter S phase. Thus, the disassembly of pre-RCs upon origin firing and their failure to re-form during S, G2 and M phases could have an important role in preventing re-replication during the cell cycle. To address how yeast cells restrict the formation of pre-RCs to a short period following anaphase, it was investigated when during the cell cycle Cdc6 protein is capable of forming pre-RCs and promoting DNA replication. It could be shown that Cdc6p is capable of inducing DNA replication only when synthesised during a narrow window of the cell cycle, between the end of anaphase, when B-type cyclins are destroyed, and a point that roughly coincides with the beginning of the subsequent S phase, when Clb5-6/Cdkl kinases are re¬ activated. There exists, therefore, a "point of no return" in late GI, after which Cdc6 protein loses the ability to promote DNA replication, presumably because it can no longer form pre-RCs. The data obtained in the experiments of the invention are consistent with the proposal that assembly of pre-RCs is inhibited by the same set of cyclin B/Cdkl kinases that trigger DNA replication (Dahmann et al . , 1995) . In the experiments of the present invention the following results have been obtained: 1) Cdc6p synthesis is able to promote formation of pre-RCs in α factor but not in nocodazole arrested cells It was shown that de novo synthesis of Cdc6p, either as cells exit from mitosis or subsequently in late GI, is necessary for DNA replication in budding yeast but not for progress through other aspects of the cell cycle. Cells that fail to synthesise Cdc6 during GI proceed with the formation of mitotic spindles and segregate their unreplicated chromosomes at random to mother and daughter cells (Piatti et al . , 1995) . Inactivation of mitotic Clb/Cdkl kinases (by elevating the temperature of a strain lacking CLB1 , 3 , and 4 and carrying a ts allele of CLB2) , which are required for assembly of mitotic spindles, prevented the abnormal nuclear division of cells deprived of Cdc6 but not, surprisingly, their rapid death; that is, they lost the ability to form colonies when Cdc6 and Clb2 functions were restored (see Fig. 6 in Piatti et al . , 1995) . This experiment was repeated and the DNA content of cells measured by FACS analysis during the abortive recovery period. It was found that cells were unable to replicate their chromosomes even when Cdc6 synthesis was restored half an hour before shift to the permissive temperature, which restores Clb2 function (data not shown) . These data could be explained if Cdc6 synthesis were capable of promoting DNA replication only during certain stages of the cell cycle; cells deprived of Clbl-4 might arrest at a stage of the cell cycle during which Cdc6 cannot promote the formation of pre- replication complexes (pre-RCs) . To investigate this further, the formation of pre-RCs in response to de novo Cdc6 synthesis in cells arrested in a "GI" like state by pheromone was compared with that in cells arrested in a "G2" or "M" phase like state by nocodazole. Due to the lack of Cdc6 synthesis prior to arresting the cells, these two states do not correspond to conventional GI or G2/M states. GI cells normally possess pre-RCs at replication origins and nocodazole arrested cells normally have replicated chromosomes. To perform the experiment, a strain was used whose only functional Cdc6 protein was fused to ubiquitin and was expressed from the GAL1 -10 promoter (GAL-ubiCDCfS) . The strain also contained a cdcl5 mutation that caused the arrest of cells in late anaphase by incubation at 37°C. Upon shift down to 25°C in the presence of α factor but in the absence of galactose, cells entered the next GI phase synchronously without producing Cdc6 protein. This produced a culture of unbudded cells lacking pre- RCs at the 2 μm origin (Fig. 1) . The culture was then split and one half incubated for 90 min. in medium still lacking galactose but containing nocodazole, which caused cells to arrest with buds and high levels of Clb/Cdkl kinase but with unreplicated chromosomes lacking pre-RCs. Re-activation of Cdc6 synthesis by addition of galactose induced formation of pre-RCs (as measured by the protection from DNase I DNA sequences adjacent to those bound by ORC) in the α factor arrested culture but not in the nocodazole one. The failure to detect pre-RCs in nocodazole arrested cells (Diffley et al. , 1994) is not, therefore, due to DNA replication itself. These data suggest that de novo Cdc6 synthesis cannot induce the formation pre-RCs in cells whose state resembles that of a G2 or M phase cell, even when a prior lack of Cdc6 synthesis has prevented the replication of their chromosomes. 2) Delayed Cdc6 synthesis cannot promote DNA replication The next experiment was designed to measure more precisely the cell cycle dependence of Cdc6's capacity to promote DNA replication. Synchronised populations of GI cells were generated that lack Cdc6 protein and turned on Cdc6 synthesis at will during the subsequent progression of these cells through the cell cycle. Cells were synchronised in late anaphase using a ts cdcl5 mutation. Fig. 2A shows cellular DNA contents measured by FACS analysis in the period following release from a cdcl5 arrest in cells containing an intact CDC6 gene. Cells start with a 2C DNA content (due to IC chromosome clusters/nuclei at each end of the budded cell) and most, if not all, cells replicate DNA before they complete cell division. A transient accumulation of cells with a IC DNA content could not be seen (as would have been expected if cells divided before replicating) , but instead a transient accumulation of cells with a 4C DNA content (i.e. cells with two 2C nuclei) . To use the FACS profiles to estimate the DNA contents of nuclei during this period, the fraction of cells at each time point with single or double chromosome clusters/nuclei was also measured; i.e. uninucleate (U) and binucleate (B) cells. Assuming that all 4C cells are binucleates and that all IC cells are uninucleate cells that have not yet undergone a reductional anaphase (see later) , then the percentage of nuclei that have replicated (%2C) during the first cycle following release can be calculated using the formula: %2C = 100. (2.C4 + U - Cι)/(U + 2.B) , where Ci, C2, and C4 are the fraction of cells at each time point with IC, 2C, or 4C DNA contents (see Materials and Methods) . Application of this formula shows that most nuclei have a IC DNA content during the cdcl5 arrest and duplicate their DNA within 60 min. following release. This experiment was repeated with a cdcl5 GAL-ubiCDC6 strain. First cells were synchronised in late anaphase by shifting cells to the restrictive temperature, Cdc6 synthesis was then repressed by removing galactose from the medium, and 30 min. later cells were released from their mitotic arrest (still in the absence of galactose) by lowering the temperature. Under these circumstances, cells exited from their late anaphase arrest in the absence of de novo Cdc6 synthesis and, as a consequence, they failed to enter S phase; nuclei retained a IC DNA content for at least 120 minutes following release (Fig. 2B) but they subsequently (between 210 and 240 min.) underwent a reductional anaphase to produce nuclei with DNA contents less than IC. Re-addition of galactose to the medium when cells were returned to 25°C allowed 85% of the nuclei to replicate with kinetics similar to CDC6 + cells, which demonstrates that de novo synthesis of Cdc6 from the GAL promoter at the time of cdcl5 release is sufficient to drive S phase (Fig. 2C) . These data show that formation of pre-RCs, which does not occur in the absence of Cdc6 synthesis during a cdcl5 release (Cocker et al. , 1996), is essential for entry into S phase. Then the experiment in Fig.2 was repeated varying the time of GAL-ubiCDC6 induction; that is, cells were arrested at 37°C, Cdc6 synthesis was repressed and re- induced at 0, 10, 20, 30 etc min. after release from the cdclΞ arrest. Fig. 3A shows for each time point the percentage of nuclei that replicated their DNA during the subsequent 120 min. All cells retained the ability to replicate DNA upon addition of galactose until 20 min. after the release, some of the cells lost the ability by 30 min., and few if any cells retained it by 40 min. This experiment was repeated using a strain in which the synthesis of Cdc6 (this time not fused to ubiquitin) was instead controlled by the methionine- repressible MET3 promoter. Cells again lost the ability to replicate DNA upon induction of Cdc6 synthesis 40 min. after release from the late anaphase cdclS arrest (data not shown) . These data suggest that there exists a "point of no return", between 30 and 40 min. after cdcl5 release, after which induction of CDC6 transcription is ineffective in driving DNA replication. It is interesting that this point is around the time that cells would normally have entered 5 phase, had the CDC6 gene been fully functional. The question was addressed if a cell's failure to induce DNA replication after the "point of no return" might be due to a simple failure to synthesise sufficient Cdc6 protein? It was previously shown that Cdc6 protein made in wild type cells during late GI is rapidly degraded as cells progress through the cell cycle (Piatti et al. , 1995) . Thus, Cdc6 protein might fail to accumulate in cells that have passed the "point of no return" due to an increase in the rate of Cdc6 proteolysis at this stage of the cell cycle. Therefore the previous experiment was repeated using a strain whose sole functional CDC6 gene is an HA tagged version under control of the GAL1 -10 promoter (GAL-HA3CDC6) . The HA tagged gene is fully functional because these cells proliferate normally in galactose medium. The epitope tag allowed us to follow accumulation of Cdc6 protein by Western blotting 30 min. after induction at various times following cdcl5 release. These cells also lost the ability to replicate DNA in response to galactose induction 40 min. after release (Fig. 3B) , even though they retained the ability to accumulate Cdc6 protein throughout the time course (Fig. 3C) . In fact, higher levels of HA3Cdc6 accumulated 60 min after cdclS release, by which time no cells were capable of replicating, than at 20 min. after release, at which time galactose induced most cells to replicate. Thus, an increased proteolysis of Cdc6 after the "point of no return" is not responsible for the inability of cells that have passed it to replicate DNA in response to de novo Cdc6 synthesis. 3) Determination of when Cdc6 executes its function To address whether Cdc6 is needed for DNA replication subsequent to the formation of pre-RCs, wild type and temperature sensitive cdc6-l cells were arrested in α factor at the permissive temperature, incubated at 37°C for 2 hours (under which circumstance pre-RCs disappear (Cocker et al . , 1996)) , and then released from the pheromone block at 37°C. Wild type cells replicated their DNA synchronously, whereas the cdcβ mutants replicated extremely poorly, if at all (data not shown) . Both cultures budded with similar kinetics. CDC6 cannot therefore execute its replication function during a pheromone induced GI arrest, even though cells had previously formed pre-RCs. CDC6 is therefore necessary for the formation and maintenance of pre-RCs during early GI when Cdkl is inactive but also for subsequent DNA replication when Cdkl is activated in late GI. 4) The "point of no return" depends on activation of S phase promoting CDKs The "point of no return" corresponds roughly with activation of Clnl,2/ and Clb5,6/Cdkl kinases in late GI . Dahmann et al . (1995) have postulated that Clb/Cdkl kinases have dual roles : they trigger origins that have previously formed pre-RCs to initiate DNA replication, while they simultaneously prevent de novo formation of pre-RCs. If S phase promoting Clb/Cdks were involved in preventing Cdc6 from inducing replication, then inactivation of CLB5 and CLB6 should delay the "point of no return", as they are the first B-type cyclin genes to be expressed during the cell cycle (Schwob and Nasmyth, 1993) . S phase entry is delayed by about 30 minutes in clb5 clb6 double mutants; that is, until the later accumulation of Clbl-4/Cdkl kinases (Schwob et al . , 1994) . This delay is also seen during release from a cdcl5 arrest. CLB5 CLB6 cdclΞ cells commence replication about 45 min. after their release, whereas clb5 clbβ cdcl5 mutants do not start until 90 min. even though budding and activation of Clb2/Cdkl kinase occur on schedule (Fig. 4A and B) . Deletion of CLB5 and CLBfS also extended the length of time, following release from a cdclδ arrest, during which induction of Cdc6 was capable of promoting DNA replication (compare Figs. 3A and 4C) ; 50% of CLB5 CLB6 cells passed the "point of no return" by 30 min., whereas the equivalent point was not reached until 60 min. in clJb5 clJbfS double mutants. Very similar results were obtained comparing the "points of no return" of cdcl5 MET-CDC6 and cdclδ clb5 clbβ MET-CDC6 cells. Thus, loss of Clb5 and Clb6 activity delays passage through the "point of no return" to a degree that is similar to the delay in S phase entry. This finding implies that Cln/Cdkl kinases, which are also required for S phase entry as activators of Clb kinases (Amon et al . , 1994; Dirick et al . , 1995) , are not sufficient to promote passage through the "point of no return", because the kinetics of their activation is not affected by deletion of CLB5 and 6. Clbsl-4 assume the S phase promoting function of Clbs 5 and 6 in clJb5 clJbfS double mutants. It was presumed that they also assume from Clbs5 and 6 the function of promoting passage through the "point of no return" . This presumption is consistent with the finding that inhibition of all six Clb/Cdkl kinases in nocodazole blocked cells is sufficient to induce pre- RCs (Dahmann et al . , 1995) . 5) Cdc6p interacts in vivo with Clb/Cdkl kinases A histone HI kinase activity co-immunoprecipitates with a version of Cdc6 protein tagged at its C-terminus with the HA3 epitope and expressed from its own promoter (Fig. 5A) . The Cdc6-associated kinase in extracts derived from cells of various cdc mutants incubated in either permissive or restrictive conditions was assayed and active kinase was found in all extracts except those prepared from cdc28-13 and cdc4 -l mutant cells incubated at the non-permissive temperature (Fig. 5B) . Western analysis showed that Cdc6p was present at similar levels in these extracts (Fig. 5B) . Several data suggest that the activity associated with Cdc6 corresponds to Clb/Cdkl kinases. First, these kinases are inactive in cdc4 -l cells due presumably to accumulation of p40 SIC1 protein (Schwob et al . , 1994) and deletion of STC1 restored Cdc6-associated kinase activity (Fig. 5B) . Second, little or no kinase activity associated with HA tagged Cdc6 expressed from the GAL promoter (GAL-HA3CDCf5) was found in cdc28 -4 cycling cultures (Fig. 5C) . Third, the kinase was inhibited by the addition of purified p40 SICl (Fig. 5D) . Fourth, Cdc28 protein co-immunoprecipitated with Cdc6p in extracts from GAL-HA3CDC6 cells (Fig. 5E) . These data suggest that Cdc6p made in S, G2 or M phases associates with Clb/Cdkl kinases. This association might be instrumental in preventing Cdc6 from forming pre-RCs at origins during these stages of the cell cycle. The "point of no return" might therefore correspond to the point in the cell cycle at which Cdc6 protein, if present or produced, associates with Clb/Cdkl kinases. It is not yet known whether Cdc6p is a target for these kinases in vivo ; it can, however, be phosphorylated by Clb/ but not by Cln/Cdc28 kinases in vi tro (data not shown) . 6) ORC associates with ARSs To measure the association of ORC, Cdc6, and Mcm7 with specific DNA sequences in vivo, extracts from cells treated with formaldehyde were prepared , chromatin was sheared to an average size of 500 bp, and PCR was used to measure the abundance of specific sequences bound to immunoprecipitated myc tagged proteins. Four sets of PCR primers were used to amplify three regions surrounding ARS1 and one including ARS1. A similar procedure was employed for ARS305. Both of these two origins fire once during most if not all S phases (Campbell and Newlon, 1991) . All four fragments from each locus were similarly amplified from sheared chromatin that had not been immunoprecipitated using the myc-specific antibody. In contrast, fragments containing ARS1 and ARΞ305 were preferentially amplified from immunoprecipates made from cells expressing 0rc2-myc. This preferential amplification of ARS containing fragments did not occur with immunoprecipitates prepared from cells expressing Csel- myc or from cells that did not express any myc tagged protein (Figure 7A and B) . Preferential amplification of ARS fragments also depended on treatment with formaldehyde and on the inclusion of anti-myc antibody during the immunoprecipitation procedure (Figure 7A) . The preferential amplification of ARS1 but not of ARS305 (from the same DNA prep) was abolished by a point mutation in ARSl's ACS that greatly reduces ORC binding in vitro and firing in vivo (Marahrens and Stillman, 1994; Rao and Stillman, 1995) (Figure 7C) . It was also abolished by a combination of B element mutations that reduce ARS1 function. These data demonstrate that immunoprecipitation of DNA crosslinked to 0rc2p is a reliable method of measuring ORC's occupancy of origins in vivo. They also show that ORC does not associate with many sites within chromatin. Its binding in vivo may be restricted to ARS sequences. 7) Cdc6p and Mcm7p associate with origins in a cell cycle dependent manner The same procedure was used to test whether Cdc6p and Mcm7p associate specifically with origins. ARS containing fragments but not neighbouring ones were preferentially amplified in anti-myc immunoprecipitates from cells expressing Cdc6-myc. Furthermore, the preferential amplification of ARS1 but not ARS305 sequences was abolished by ACS and B element mutations at ARS1 (Figure 7B and C) . This suggests that Cdc6p like ORC associates specifically with ARS sequences in vivo. Cdc6p's association with ARSs was measured as unbudded GI cells, isolated by elutriation, progress through the cell cycle (Figure 8A) . Association was already high in the starting culture of small unbudded cells, presumably as a consequence of the burst of Cdc6 synthesis at the end of mitosis. It remained high for much of GI, decreased at the start of S phase, remained low during G2 and metaphase, but reappeared during telophase, just before cytokinesis. This pattern is similar to that of the Cdc6 dependent footprint at origins (Diffley et al . , 1994) . The latter has never been measured in elutriated cells, so that a direct comparison is not possible. A similar set of experiments using cells expressing Mcm7-myc (Figs 7B, 7C, and 8B) show that Mcm7p associates with ARS1 and ARS305 but not their adjacent sequences throughout GI. Mcm7p leaves origins during S phase, which is somewhat later than Cdc6p, and does not reappear until telophase. This pattern of association is paralleled by Mcm7p's accumulation within nuclei. It accumulates within nuclei during telophase, remains there during GI, but accumulates in the cytoplasm during G2 and M phase (Dalton and Whitbread, 1995; Figure 8B) . 8) Association of Mcm7p with origins depends on Cdc6p The cell cycle profile of Mcm7p's association with origins resembles that of Cdc6p. Unlike ORC which is bound to origins throughout the cell cycle, both Cdc6p and Mcm7p are bound mainly only during GI . Cdc6p is rapidly degraded during G2 and M phases but due to transcriptional activation by Swi5 and Ace2 is synthesized de novo as cells complete mitosis (Piatti et al . , 1995) . The abrupt re-accumulation of Cdc6p at the end of mitosis presumably accounts for its arrival at ARSs during telophase. Might it also be responsible for facilitating the association of Mcm7p with ARS sequences at this stage of the cell cycle? A clue that it might be comes from the recent observation that loading of Mcm3p onto chromatin in Xenopus extracts is abolished by immunodepletion of Cdc6p (Coleman et al . , 1996) . However, this study did not exclude the possibilty that immuno-depletion of Cdc6p depleted additional factors needed for Mcm3p' loading onto chromatin. To establish whether the loading of Mcm7p onto chromosomes and their origins in yeast is dependent on Cdc6p, Mcm7-myc expressing strain was constructed whose endogenous CDC6 gene had been deleted (Dcdc6) and replaced (albeit at another locus) by a ubiquitin- CDC6 fusion under control of the GAL promoter ( GAL- ubi -CDC6) . Cells from this strain synthesize Cdc6p and replicate DNA only in the presence of galactose (Piatti et al . , 1996) . From a culture that had been shifted 75 minutes previously from galactose to raffinose, by centrifugal elutriation a population of unbudded GI cells lacking Cdc6p was isolated, which was then split into two and one half incubated in the presence and the other half incubated in the absence of galactose (Figure 9) . Unlike wild type cells treated in the same way, little or no Mcm7p could be detected at ARS1 or at ARS305 in the starting culture of unbudded GI cells (0 min) . This state of affairs continued as cells grown in the absence of galactose progressed through the cell cycle. As expected, these cells failed to replicate their DNA. However, in the culture grown in the presence of galactose, Mcm7p appeared within 20 min at ARS1 and ARS305. It persisted at these origins until cells entered S phase and then disappeared (during G2 and M phases) despite the continued synthesis of Cdc6p. The amount of Mcm7p bound to chromosomes in these two populations was also compared. Mcm7p was detected on chromosomes within 30 min after galactose induction and it persisted until late GI or the beginning of S-phase. In contrast, it could not be detected at any time point on chromosomes isolated from cells grown in the absence of galactose. It could therefore be concluded that de novo synthesis of Cdc6p is essential for the loading of Mcm7p onto yeast chromosomes and their origins. Measuring also the cellular location of Mcm7-myc in the two cultures by in situ immunofluorescence allowed to evaluate whether Cdc6p is directly needed for loading Mcm7p onto chromosomal origins or merely required for it to enter nuclei. Mcm7p was concentrated within nuclei of most cells in the starting population of GI cells which lacked Cdc6p, though to somewhat lower extent compared to wild type cells (Figure 9) . Cdc6p is therefore not required for the accumulation of Mcm7p within nuclei that coincides with Cdc6p synthesis during telophase. Furthermore, induction with galactose for 20 min had little difference on the distribution of Mcm7p between nucleus and cytoplasm despite it having caused Mcm7p to load onto origins . Accumulation of Mcm7p within nuclei is not sufficient for its loading onto origins; Cdc6p is required to facilitate loading onto origins of Mcm7p already present within nuclei. From the above results, essentially the following conclusions can be drawn: 1) Cdcδ's ability to promote formation of pre-RCs and DNA replication is restricted to the GI phase of the cell cycle De novo Cdc6 protein synthesis is required for a change in the chromatin structure of origins of DNA replication, which normally occurs as cells exit from mitosis and enter GI (Cocker et al . , 1996) . The pattern of DNasel sensitivity surrounding origins suggests that the Origin Recognition Complex (ORC) binds to origins throughout the cell cycle but that another factor, possibly Cdc6 itself, joins it in the interval between exit from mitosis and re-initiation of DNA replication in the subsequent cycle (Diffley et al . , 1994) . Cdc6 protein has been shown to interact with ORC in vitro (Liang et al. , 1995) . Cdc6 is required for the initiation of DNA replication but it is not required for other aspects of cell cycle progression such as the formation of mitotic spindles or even the movement on these spindles of unreplicated chromosomes to opposite poles of the cell (Piatti et al.,1995) . These findings suggest that a change in the state of replication origins mediated by Cdc6 synthesis at the end of mitosis or later during GI is essential for the subsequent initiation of DNA replication triggered by activation of Clb/Cdkl kinases in late GI. The initiation of DNA replication in yeast can therefore be viewed as a two step process: formation at origins of pre-replication complexes (pre-RCs) , driven by Cdc6 synthesis, followed by activation of Clb/Cdkl kinases due to destruction of the Clb-specific Cdk inhibitor p40 SJCI (Schwob et al . , 1994) . The experiments of the invention address whether these two processes must occur in the correct order. Clb/Cdkl kinases are activated on schedule even when Cdc6 protein is not made during GI . What happens then when Cdc6 synthesis is restored to cells that have already activated Clb/Cdkl kinases? Is this effective in promoting DNA replication? The effectiveness of Cdc6 synthesis at different cell cycle stages in promoting DNA replication was investigated and a "point of no return" was discovered after which de novo Cdc6 synthesis fails to drive DNA replication. This "point of no return" is delayed by inactivation of Clb5 and Clb6, which are normally the first Clbs to appear during the cell cycle. Passage through the "point of no return" did not, however, depend on activity of the Cdc7 protein kinase (data not shown) , which is also necessary for initiation of DNA replication and for the switch of origins from a pre-replicative to a post-replicative state. The data suggest that the "point of no return" corresponds to a point in the cell cycle after which Cdc6 protein is no longer able to drive the formation of pre-RCs at origins of DNA replication and that the crucial event responsible for this switch in cell cycle state is the activation of Clb/Cdkl kinases. The "point of no return" not only coincides with Clb/Cdkl activation but is also delayed when their activation is delayed. Dahmann et al . (1995) showed that inactivation of Clb/Cdkl kinases in cells arrested in nocodazole by overproduction of the p40 SJCI Cdk inhibitor is sufficient to induce the formation of pre-RCs at origins. As a result, they proposed that Clb/Cdkl kinases have two opposing roles in the initiation of S phase: to inhibit the de novo formation of pre-RCs and to trigger initiation only from those origins that have previously formed pre-RCs. The re-replication of S. pombe cells either defective for Cdc2 or the B-type cyclin Cdcl3, or overproducing the Cdcl3/Cdc2 kinase inhibitor Ruml, is consistent with this hypothesis (Broek et al . , 1991; Hayles et al . , 1994; Moreno and Nurse, 1994) . According to this model, activation of Clb/Cdkl kinases prior to the formation of pre-RCs (i.e. prior to the de novo synthesis of Cdc6 protein) would finesse the system and prevent initiation of DNA replication. The results described above show that the timing of Cdc6 synthesis relative to Clb/Cdkl activation is indeed crucial. The "point of no return" may correspond to the point in the cell cycle when Clb/Cdkl kinases become sufficiently active to inhibit the formation of pre-RCs at origins. The experiments of the invention concentrated on the consequences of delaying Cdc6 synthesis relative to Clb/Cdkl activation. However, the hypothesis that Clb/Cdkl kinases inhibit the formation of pre-RCs predicts disaster not only when Cdc6 synthesis is delayed but also when Clb/Cdkl kinases are activated prematurely. In other experiments the effects on replication of deleting the SICl gene, which encodes an inhibitor of Clb/Cdkl kinases that delays their activation during GI, have been analysed, sicl mutants initiate DNA replication somewhat earlier than wild type cells but proceed through S phase more slowly. Furthermore, chromosomes and mini-chromosomes are highly unstable in sicl mutants and this defect is suppressed either by deletion of CLB5 and CLB6 or by the addition of extra origins to the chromosomes. Thus, premature activation of Clb/Cdkl kinases has a similar effect to delaying Cdc6 synthesis: it also reduces the efficiency with which origins fire. 2) Factors that prevent the formation of pre-RCs during G2 The lack of DNA replication in cells that express GAL- CDC6 only after they have passed the "point of no return" is not due to their failure to accumulate sufficient Cdc6 protein; induction of GAL-CDC6 causes more Cdc6p to accumulate in cells that have long passed this point than in those that have not (Fig. 3C) . Cdc6p is an unstable protein and it normally disappears from cells during S and G2 phases, but this is primarily due to control of CDC6 transcription. Under conditions in which Cdc6 was synthesised continuously and protein levels remained constant throughout the cycle (due to expression from the GAL promoter) , it could be noticed that more Cdc6 protein accumulated within nuclei of post anaphase and early GI cells than during S, G2, and M phases (Piatti et al. , 1996) . Rapid proteolysis of Cdc6 seems partly responsible for this pattern because Cdc6 protein accumulated to high levels within nuclei at most stages of the cell cycle in cycling cultures of cdc4 mutants, which are defective in Cdc6 degradation (Piatti et al . , 1996) . The cell cycle control of Cdc6 ' s cellular distribution resembles that of Cdc46/Mcm proteins, which are also needed for the firing of origins and which also accumulate in the cytoplasm during G2. It is doubtful, however, whether Cdc6"s failure to function in cells past the "point of no return" is due to its insufficient accumulation within nuclei . Cdc6 protein is never actually excluded from nuclei in CDC4 + cells and it accumulates to very high levels within G2 nuclei of cdc4 mutant cells growing at 25°C without bypassing the block that prevents re-replication until cells have undergone anaphase (Piatti et al . , 1996) . The results of the experiments raise the question as to why Cdc6 is only synthesised during GI and is rapidly degraded in wild type cells. This seems to be a conserved feature of the fungal cell cycle, because Cdclδp levels in S.pombe are regulated in a similar manner (Nishitani and Nurse, 1995; Muzi-Falconi et al . , 1996) . It has been shown that the block to re-replication does not depend on Cdc6 ' s absence from the nucleus during G2. It might nevertheless contribute to the fidelity with which such cells prevent re-replication. Cdc6 ' s inability to promote DNA replication in cells that have past the "point of no return" could be due either to its association with inhibitory factors or due to its inhibition by modification. The finding that Cdc6 associates with Clb/Cdkl kinases could be important with this regard. Phosphorylation of Cdc6 by these kinases might prevent it from forming pre-RCs. Alternatively, Clb/Cdkl kinases associated with Cdc6 might inhibit, through phosphorylation, other Cdc6- interacting proteins needed for pre-RC formation (for example, members of ORC) . The failure of cells past the "point of no return" to form pre-RCs and to promote replication in response to de novo Cdc6 synthesis could also stem from the inactivity in these cells of other proteins needed to form pre-RCs or to initiate DNA replication from them. For example, proteins of the Cdc46/Mcm class, which are essential for origin firing, are found in the cytoplasm during G2. The data obtained in the experiments of the present invention also confirm that a drop in the concentration of Mem proteins within nuclei or a reduction in their ability to bind chromatin could contribute to the "point of no return" . 3) Factors that drive the eukaryotic replication cycle A key requirement for the mechanism that drives chromosome duplication is that it should permit replication origins to fire once and once only once between succeeding rounds of sister chromatid segregation. Bacteria must also restrict replication to once every mass doubling, but the mechanism by which they do this is different due to their possession of only a single origin of DNA replication. Imperfections in the bacterial firing mechanism are not disastrous because unscheduled initiation from a single origin leads to replication of the entire genome; that is, it does not lead to over or under representation of genes. The downside of this simple device is that replication can take longer than mass doubling and this problem can only be solved by re-initiating replication before sister chromatids from the previous round have been segregated. The mechanism used by eukaryotes whose genomes are carried on multiple chromosomes, each of which are replicated from multiple origins, must therefore be much more efficient than that used by bacteria. There has been much speculation as to how eukaryotic cells might achieve an efficient "once only" firing device, but few hard facts. The current knowledge about replication in S . cerevisiae is sufficient to assemble, for the first time, the crude outlines of the device actually used (see Fig. 6 and Dahman et al . , 1995) . Initiation is from defined sites and depends on two types of factors: those like ORC and Cdc6 and Cdc46/Mcm proteins that bind to origins or the sequences that surround them and S phase promoting factors like Clb/Cdkl and Cdc7 kinases, whose activation in late GI actually triggers the initiation of DNA replication. Origins and their surrounding chromatin exist in two states: a post- replicative one, in which they are bound by ORC alone, and a pre-replicative one, in which they are bound by ORC and also Cdc6 and Mem proteins, as assumed by Diffley et al. , 1994; Cocker et al. , 1996. The latter is referred to as the pre-replication or pre-initiation complex. Only origins in a pre-replicative state can be triggered to initiate DNA replication by Clb/Cdkl kinases. Key aspects of this "once only" replication device are first the mechanisms that govern the transitions between the two states of origins and second the mechanisms that generate sharp fluctuations in the activity of Clb/Cdkl kinases. It is proposed that pre-RCs are destroyed either by initiation itself or by replication through them and that the formation of new pre-RCs is inhibited by the very same set of Clb/Cdkl kinases that trigger initiation. According to this scheme, the replication cycle is driven by a Clb/Cdkl cycle. It starts with a period of low Clb/Cdkl activity (GI) , which permits formation of pre-RCs. Activation of Clb/Cdkl kinases triggers replication from pre-RCs that had formed during the previous period of low Clb/Cdkl activity and simultaneously blocks formation of any new pre-RCs (S phase) . The subsequent period of high Clb/Cdkl kinase activity (G2/M phases) maintains the block to the formation of pre-RCs. The cycle is completed by the final inactivation of Clb/Cdkl kinases and re-entry into the low kinase state, which is mediated, at least in part, by the same complex (the APC) that promotes the segregation of sister chromatids (Irniger et al . , 1995) . The antagonistic effects of Clb/Cdkl kinases on initiation ensure that there is no stage during this cycle in which cells can both form pre-RCs and trigger initiation of DNA replication from them. Involvement of the APC in both cyclin destruction and in promoting sister chromatid separation may be key to linking re- replication with chromosome disjunction. The momentum of the device, i.e. successive rounds of DNA replication, can only be maintained by fluctuations in the activity of Clb/Cdkl kinases. It is a reciprocating device in which motion (i.e. rounds of replication) is driven by the alternate inactivation and re-activation of Clb/Cdkl kinases. In this sense, it is deeply analogous to the reciprocating steam engine, whose steam corresponds to kinase activity and whose piston corresponds to origins. Steam, i.e. kinase, can only do work by driving the piston upwards (i.e. drive replication) if it (i.e. origins) has previously returned to the down state (i.e. contains pre-RCs) . Passage from the up state (an origin that has initiated replication) depends on evacuation of the steam (i.e. inactivation of kinases) from the chamber containing the piston (i.e. the cell). The essence of the reciprocating steam engine is that entry of steam into the chamber only performs work if the piston has been returned to the down state by previous evacuation of steam from the chamber. Likewise, Clb/Cdkl kinases only drive replication when their prior inactivation has permitted the formation of pre-RCs. Just as the piston can only move once during a cycle of expansion and contraction, so too can origins only fire only once during a cycle of kinase activation and inactivation. If then, there exists a cell cycle engine (Murray and Hunt, 1993), this surely could be a key part of it. Timing is crucial to the operation of the "reciprocating" steam engine. The implication of the finding that Cdc6 must be synthesised before Clb/Cdkl kinases are re-activated suggests that timing is also crucial to the "reciprocating" replication cycle in S . cerevisiae. It was an object of the invention to provide compounds for use in cancer therapy. The experiments of the present invention have shown that the initation of DNA replication in eukaryotic cells, exemplified with yeast cells, is a two step process: first the formation at future replication origins of pre-replication complexes and second the activation of S phase promoting cyclin dependent kinases (Diffley et al. , 1994; Schwob et al. , 1994; Cocker et al. , 1996) . Synthesis of the unstable protein Cdc6 is essential for the formation of pre-RCs at yeast origins and for subsequent initiation of DNA replication. The present invention is based on results of an investigation of when during the cell cycle synthesis of Cdc6 is effective in forming pre-replication complexes and thereby in driving a new round of DNA replication. The experiments of the present invention have shown that Cdc6 must be synthesised during the period of the cell cycle in which S phase and M phase promoting cyclin dependent kinases are inactive (i.e. GI phase) . It has been shown that synthesis of Cdc6 after these kinases have become active is ineffective in driving a new round of DNA replication. Cells deprived of Cdc6 proceed with the activation of S and M phase promoting kinases and enter mitosis and even attempt to undergo anaphase without having undergone DNA replication, which is a lethal event (Kelly et al . , 1993; Piatti et al . , 1995) . The key conclusion that can be drawn from these results is that a temporary interference with Cdc6 function, during a cell cycle period in which S or M phase promoting cyclin dependent kinases become active, is sufficient to kill proliferating cells but has no effect on quiescent cells. The present invention provides a method of interfering with the proliferation of tumour cells. This method is based on a novel concept which allows for identifying drugs which are toxic to the tumour cells, i.e. rapidly growing cells, but which are harmless to the non- dividing or slowly dividing non-tumour cells of the patient being treated with that drug. The present invention provides a method to kill tumour cells which is based on the utilization of this novel observation. While it has been known since the first CDC6 mutations were isolated (Hartwell, 1976) , that CDC6 is essential for proper DNA replication and a loss of function of the CDC6 gene results in cell cycle arrest, the experiments of the present invention show for the first time that an interference with Cdc6 function only for a short period of time is also a lethal event for cells that are in the process of activating cyclin dependent kinases (Cdks) that drive DNA replication and mitosis. The experiments of the invention also suggest a mechanism for this phenomenon. Thus, the application of a drug that interferes with the ability of Cdc6 to form or maintain pre-RCs (or a drug that destroys pre-RCs) is lethal to cells which proceed to activate S and M phase promoting Cdks in the presence of the drug, even when the drug has been removed) . The repeated temporary application, e.g. for 12 to 24 hours, of drugs that interfere with the ability of CDC6 to form or maintain pre-replication complexes will be lethal to rapidly growing tumour cells which activate S and M phase promoting cyclin dependent kinases in the presence of the drug, but will have little or no effect on quiescent or even slowing dividing host cells which spend much longer in a GI state where these kinases are inactive. The results of the experiments of the present invention provide a proof of this principle in the budding yeast Saccharomyces cerevisiae . It has been demonstrated that depriving cells of Cdc6 function merely for 40 minutes, between their exiting mitosis and re-activating S phase promoting Cdks in the next cycle, is a lethal event. Since most of the components directly involved in the initiation of DNA replication, for example, S phase promoting cyclin dependent kinases, the origin recognition complex (ORC) , Cdc6, and Mem proteins, which are DNA binding proteins, have proven to be highly conserved between fungi and animal cells, this proof of principle in budding yeast according to the results of the experiments of the present invention is highly relevant to human cells. The present invention is based on the validity of the assumption that the principles about Cdc6 function and the assembly of pre- replication complexes, or their inhibition, respectively, that have been discovered in yeast are also valid for human cells. The identification of homologous genes in as distantly related organisms as yeast and Xenopus has so far always led to the identification of a homologous mammalian gene. Therefore, a recent report on the Xenopus homologue of Cdc6 (Dunphy, W. , Meeting "The Cell Cycle", Cold Spring Harbour, May 1996) is a proof that DNA replication in higher eukaryotic cells, and also in human cells, also depends on the presence and activity of Cdc6 during GI phase when S and M phase promoting Cdks are inactive. Temporary inhibition of the cell's ability to assemble pre-replication complexes is therefore as lethal to rapidly dividing human cells as it is to proliferating yeast cells. Replication origins have not yet been fully characterized in mammalian cells and may prove to differ, in certain aspects, to yeast origins. Nevertheless, based on the reported conservation of replication mechanisms in eukaryotic cells, it has to be assumed that they will operate according to similar principles that involve homologous protein functions. The invention provides the use of compounds which interfere with the function of cdc6p to form or maintain pre-replication complexes in an animal cell without impairing the cell's ability to activate cyclin dependent kinases that promote S phase and/or M phase, for the treatment of cancer. In the following, compounds which interfere with the function of cdc6p to form or maintain pre-replication complexes, are also designated "cdc6p inhibitors". In exploitation of the above-described concepts, the present invention further provides screening methods for identifying compounds for treating cancer that can interfere with Cdc6-mediated pre-replication complex formation and maintenance in animal cells. In one embodiment of the invention, this screening method is based on the detection of the formation of pre-replication complexes. An example is an assay in which the binding of Cdc6 protein to chromatin is detected by antibodies directed against Cdc6. The yeast cdc6 protein having been cloned (Zhou et al . , 1989) , based on the published yeast sequence the homologous human protein can be obtained by methods known in the art, e.g. by screening human cDNA libraries with DNA probes derived from the yeast sequence or by PCR techniques. An alternative method to obtain the human CDC6 cDNA is the functional complementation of the cdc6(ts) allele in yeast by expression of the homologous human CDNA according to methods kown in art, designated "expression cloning". The human CDC6 cDNA may then be expressed according to known methods in suitable hosts (Maniatis et al . , 1982) . The thus obtained human cdc6 protein or peptides derived from its amino acid sequence may then be used to obtain antibodies according to known methods, preferably monoclonal antibodies, which are produced by conventional hybridoma technology (see e.g. Harlow and Lane, 1988) . It has been shown in the experiments of the present invention that Cdc6p and Mcm7p associate specifically with replication origins during GI but not during G2 in S. cerevisiae. Mcm7's association with origins depends on Cdc6. Since the binding of Cdc6 has been proven to be a pre¬ requisite for the binding of Mem proteins to chromatin, Cdc6 activity may also be measured indirectly by measuring the binding of Mem proteins to chromatin. Such an assay may be based on the measurement by in si tu immunofluorescence of the amount of mem protein bound to chromatin that has been spread and fixed. An assay for identifying cdc6p inhibitors by detecting the formation of pre-replication complexes may be performed using live human cells. To this end, rapidly dividing cells, e.g. cells from a tumour cell line, are grown and incubated with the test substance for a period of time sufficient for binding of the protein(s) to chromatin. Then the cells are lysed, fixed, e.g. on cover slips and the binding of cdc6p and/or mem proteins to chromatin is determined as described above. The use of synchronized cells, which are at a stage of the cell cycle prior to cdc6p synthesis, is advantageous. Since the drugs of interest must be specific for an activity that is present only during a narrow window of the cell cycle, an in vi tro system which provides this very window is also useful for identifying cdc6p inhibitors. Such a screening assay may be performed as a microtitre plate assay, in which a Xenopus laevis egg extract (Blow,J.J. and Laskey, R.A. , 1988) , which is capable of DNA replication and therefore also contains cdc6p, and chromatin, e.g. Xenopus laevis sperm chromatin, is incubated, in the presence of the test substance, for a period of time sufficient for cdc6p to form pre-replication complexes. After the incubation period assembly of Cdc6 protein onto chromatin can be measured by simple immunological methods, e.g. in si tu immunofluorescence, employing anti-cdc6 antibodies carrying an immunofluorescent label . References for X.L. extract capable of replication. The use of Xenopus components in an assay system is extremely useful in proving the potentional drug's efficacy in animal cells. Compounds identified as inhibitors of cdc6p's binding to chromatin in a screening assay using the in vi tro Xenopus laevis system can be rapidly tested on Xenopus tissue culture cells to confirm their effect on rapidly cycling somatic cells. It would, however, be preferable to test compounds using an in vi tro assay for the formation of pre-RCs in extracts from human cells. To do this, sperm chromatin or nuclei isolated from GI cells would be added to extracts prepared from human cells that express high levels of cdc6p and a cyclin dependent kinase inhibitor such as P21 or P27. If the human protein were found to rescue the lethality of a yeast cdc6 mutation, then yeast cells can be employed to assay the function of the human cdc6 protein in yeast cells. An example for a yeast based screening assay for identifying cdc6p inhibitors, which can be used according to the invention, is based on the sort of experiments conducted by Liang et al . , 1995, who have shown that cdc6 is a multisuppressor of ore mutations in yeast, i.e. the cells carrying an ore mutation require overexpression of cdc6p for their growth. Thus, Saccharomyces cerevisiae cells that carry an ore mutation, e.g. the orc-5 or orc-2 mutants described by Liang et al . 1995, and an inducible, e.g. galactose inducible, plasmid containing the, CDC6 sequence (preferably the human sequence) such that they overexpress cdc6p, may be used in a screening assay. The yeast cells are grown and incubated with the test substances. Substances that cause the cells to die are candidates for inhibitors of cdc6 function. Another means of assaying the function of cdc6 homologues (and thereby screening for drugs that interfere with that function) is based on the ability of CDC6/cdcl8 homologues to induce multiple rounds of DNA replication, in the absence of mitosis, in the fission yeast Schizosaccharomyces pombe (Muzi-Falconi, et al . , 1996, Nishitani und Nurse, 1995) . Inhibition of cdc6 function by a test substance will abolish the increase in DNA content, which can be measured by conventional means, e.g. by staining with DAPI (4,6- diamido-2-phenylindole-dihydrochloride) . To narrow the results of the above screening methods to drugs which, while blocking cdc6 function, do not impair the activity of cyclin dependent kinases, it is necessary to do controls. Suitable controls show that these kinases are still active and can be activated in the presence of the drug that interferes with cdc6 function. Such control assays may be based on the detection of kinase activity (CDK2) , e.g. by HI histone phosphorylation, and/or the accumulation of these kinases' relevant cyclins (Cyclins E and A) with anti- cyclin antibodies carrying an immunofluorescent label. The controls may be carried out in parallel with the primary screen. Preferably such controls are carried out in a second screening step. Chemical compounds that inhibit the binding of cdc6 to chromatin while they retain kinase activity and/or do not prevent the cells from accumulating the relevant cyclins (ClnE, ClnE) fulfil the requirement of blocking cdc6 activity while leaving kinase function and activation unimpaired. It is expected that the screening methods of the invention that are based on the detection of pre-RCs will identify not only drugs that interfere with cdc6p function, but also drugs that cause S phase promoting Cdks to become active prematurely. Such drugs are expected to interfere with Cdk inhibitory proteins and might be equally toxic to rapidly dividing tumor cells, because the premature activation of Cdks would also interfere with pre-RCs. In parallel to drug screening, it is useful to conduct the sort of experiments of the present invention, that were performed with yeast, in animal cells. This may be accomplished by replacing the endogenous CdC6 gene of mouse embryonic stem cells by a version of the gene whose transcription can be induced, i.e. be turned on and off at will, for example using the tetracycline system. A prerequisite for the feasibility of this type of experiment is that there proves to be a single copy of the mouse Cdc6 gene. This can be verified by standard methods such as Southern hybridization. It has to be borne in mind that the drugs identified according to the invention are inhibitors against cdc6 function and may thus be toxic to other cells that are, at the time the drug is exerting its effect, performing a rapid cell cycle. To prevent undesired side effects on such cells, i.e. stem cells, particularly those from bone marrow, like erythrocyte and thrombocyte progenitors, an additional drug may have to be applied that interferes with signalling pathways specific for such cells such that the activation of S phase promoting kinases is prevented without affecting tumour cells, and the non-tumour cells are arrested in GI phase of the cell cycle, while the cdc6p inhibitor does its effect. Treatment of a patient with both drugs would block stem cells in a low kinase state and would enable them to assemble pre-RCs once the anti-Cdc6 drug is no longer active, but it would not compromise the lethality of the anti-cdc6 drug to inhibit pre-RCs in tumour cells. A possible example would be to use immunosuppressant drugs like cyclosporin which are, due to their mode of action, likely to be more effective in blocking bone marrow cells in a low kinase state than they are in blocking tumour cells in such a state. This concept of protecting normal cells has been proven in the yeast experiments of the invention which showed that blocking yeast cells in a low cyclin dependent kinase state with α-factor prevents cells from passing the "point of no return" and thereby alleviates the lethality associated with blocking synthesis of Cdc6. Brief description of the drawings Fig. 1: Pre-RCs cannot be formed in G2/M Fig. 2: Cells deprived of Cdc6 in late mitosis are unable to replicate in the next cell cycle but undergo reductional" anaphase Fig. 3: Cells pass a "point of no return" at the time of S phase entry after which Cdc6 synthesis is unable to promote DNA replication Fig. 4: Deletion of CLB5 and CLB6 causes a delay in S phase entry and a similar delay in the "point of no return" Fig. 5: B-type cyclin dependent Cdc28 kinase associates with Cdc6p in vivo during S, G2 and M phases. Fig. 6: The pre-RC/Cdkl cycle in yeast Fig. 7: In vivo Associations of 0rc2, Cdc6 and Mcm7 Proteins with ARSs Are Detected by Formaldehyde-crosslinking and a Subsequent Immunoprecipitation Fig. 8: Cell Cycle-regulation of the Association of Cdc6p and Mcm7p with ARSs Fig. 9: Association of Mcm7p with ARSs and Chromatin Depends on the Presence of Cdc6p If not otherwise indicated, the following materials and methods were used in the Examples: i) Strains, media and reagents All yeast strains were derivatives of or were backcrossed at least three times to W303 (HMLa, HMRa, ho, ade2-l , trpl -1 , leu2-3 , 112, his3 -ll , 15, ura3 , ssdl ) . Cells were grown in YEP medium (1% yeast extract, 2% bactopeptone, 50 mg/1 adenine) supplemented as indicated with 2% glucose (YEPD) , 2% raffinose (YEPR) or 2% raffinose +2% galactose (YEPRG) , except for the experiment described in Example 2 B and C, where 0.1% galactose was added to YEPR medium. The synthetic medium lacking methionine (-Met medium) is yeast nitrogen base (0.8%) supplemented with amino acids, adenine, uracil and 2% glucose. ii) Plasmid constructions and genetic manipulations Standard techniques (Mortimer and Hawthorne, 1969) were used for genetic crosses and DNA manipulations (Maniatis et al . , 1982) . To generate the GAL-ubiCDC6 construct (C2835) , the ATG of CDC6 has been replaced by a BamHI site introduced by PCR. Afterwards, a 2 kb BamHI/ (Ndel)Hindlll fragment of CDC6 has been used to replace the MCM1 fragment of a GAL-ubiR-MCMl fusion in YIplac211 (a kind gift from Gustav Ammerer) . The resulting plasmid has been cut with Apal for integration in single copy at the URA3 locus of W303. This strain was then crossed to K4055 ( cdc6 : :hisG, trpl : : TRPl MET-CDC6, Piatti et al. , 1995) to generate, after sporulation, strain K4675 ( cdc6 : :hisG, ura3 : : URA3 GAL-ubiCDC6) . K4675 was crossed to a cdcl5-2 strain (K1994) or to a cdc7-l strain (K2033) to generate, after sporulation, strains K5032 {MATa , cdcl5-2 , cdcβ : :hisG, ura3 : : URA3 GAL-ubiCDC6) , K5033 (MATa, cdcl5-2, cdc6 : :hisG, ura3 : : URA3 GAL- ubiCDCβ) and K5029 (MATa, cdc7-l , cdc6 : :hisG, ura3 : : URA3 GAL-ubiCDC6) . The C-terminal HA3 tagged version of CDC6 used here is similar to the one described before (Piatti et al . , 1995) , with the exception that it does not contain His tags (plasmid C2838) . C2838 was integrated in single copy at the CDC6 locus of W303 to generate a full length tagged version of CDC6 flanked by a truncated untagged version of the gene (strain K4528) . K4528 was crossed to various cdc mutants to generate, after sporulation of the corresponding diploids, the strains used in Example 5) . The GAL-HA3CDC6 (C2837) construct was made by inserting the triple HA NotI cassette (Tyers et al . , 1992) after the ATG of CDC6 , where a NotI site was introduced by PCR. The tagged gene (ending at the Ndel site of CDC6) was then cloned in Yiplac211 downstream of a GAL1 -10 promoter containing 70 bp of CLB5 leader sequence. Subsequently, the plasmid was cut with either Bell for integration at the CDC6 locus (in one copy, strain K5095, or in five copies, strain K4527) , or StuI for integration at CXRA3 (strain K5761) . K5761 was crossed to K4143 ( cdc!5-2, cdcβ : :hisG, trpl : : TRPl MET- CDC6) to generate, after sporulation, strain K5763 ( cdcl5-2, cdc6 : :hisG, ura3 : : URA3 GAL-HA3CDC6) . iii) Cell Synchronization Techniques To perform cdcl5-2 block/release experiments, cells were grown to exponential phase, filtered and inoculated into prewarmed medium. Cell cycle arrest was obtained by 3 hours incubation at 37°C and then cells were filtered again and released at 25°C. In the cases where CDC6 synthesis is controlled by the GAL1 -10 promoter ( GAL-ubiCDC6 or GAL-HA3CDC6) , cells were harvested by filtration and incubated in prewarmed YEPR medium at 37 C C for 30 minutes (GAL promoter off) before release in YEPR at 25°C. For induction 2% galactose (or 0.1% in the experiment described in Example 3B and C) was added to aliquots of cells at the different time points. In the cases where CDC6 expression was driven by the MET3 promoter (MET-CDC6) , cells were treated as above with the exception that the permissive and non- permissive conditions for CDC6 synthesis were obtained by the use of respectively -Met medium and YEPD+2% methionine. Due to a cell separation defect displayed after release from the cdc!5 block, some of the cells start replicating before cell division, producing a 4C peak in the FACS profiles, while some others divide before entering S phase. Cells with a 4C DNA content must be binucleate, while the ones with IC DNA content must be uninucleate. Cells with 2C DNA content can be either bi- or uninucleate. Therefore, to quantify more accurately the fraction of cells able to undergo DNA replication, at each time point after release (up to 2 hours) the fraction of cells was scored by FACS with IC, 2C or 4C DNA contents and on the same cell samples the fraction of uni- versus binucleated cells (as determined by immunofluorescence on the propidium iodide stained cells) . This allowed us to determine the percentage of 2C nuclei (i.e. nuclei which have replicated) by using the following equation: %2C = 100. (2.C4 + U - Ci) / (U + 2.B) , where C " ι, C2, and C4 are the fraction of cells at each point with IC, 2C, or 4C DNA contents, and U and B the fractions of uni- and binucleated cells respectively . It can be assumed that the result of this equation reflects reliably the fraction of nuclei which have replicated during the S phase immediately after cdcl5 release, but its application during later stages of the cell cycle has some limitations: in fact, after completing S phase a fraction of cells undergoes mitosis before having completed the previous cell separation, producing cells with more than 2 nuclei. Since these cells have been scored as binucleates for simplicity, when these cells start appearing the percentage of 2C nuclei becomes overestimated, because some of the cells with a 4C DNA content have four IC nuclei rather than two 2C nuclei. Another limitation arises from the feature of Cdc6 deprived cells to undergo anaphase in the absence of DNA replication: in this case, the assumption that IC cells must be uninucleate does no longer hold true. In cells lacking Cdc6, calculations are even more complicated by the appearance of cells with <1C DNA contents. Therefore, if the formula described above is applied to these cells, the result can be a negative number. In conclusion, it can be said that the above equation can be reliably applied only to estimate the fraction of nuclei which have undergone DNA replication during the first S phase after cdcl5 release. For nocodazole arrest/release experiments described in Fig. 6A and B, cells exponentially growing in YEPRG were incubated with 5mg/ml nocodazole (and a final concentration of 1% DMSO) at 25°C for 2.5 hours. Cells were then filtered, washed with 3 volumes of YEPR +1% DMSO and incubated in either YEPR +2 mg/ml α factor at 25°C (Fig. 6A) or YEPR at 37°C (Fig. 6B) for 3 hours. Afterwards, cultures were split in two and one half was incubated in the presence of 2% galactose for 15 minutes. Subsequently, cells were released by filtration from the α factor or the cdc7 blocks into either YEPRG or YEPR medium at 25°C for 2 hours. iv) In vivo footprinting K5033 cells (MATa, cdcl5-2, cdcβ : :hisG, ura3 : : URA3 GAL- ubiCDCβ) exponentially growing in YEPRG were arrested by 3 hours incubation at 37°C and then released into YEPD+α factor (lOmg/ml) for 90 minutes. An aliquot of the culture was then transferred into YEPRG+α factor, while another was released from α factor into YEPD+nocodazole (20 mg/ml) . After 90 minutes cell samples were withdrawn for in vivo footprinting and part of the culture containing nocodazole was transferred to YEPRG+nocodazole for 90 minutes. From each condition footprinting experiments on the 2m origin have been performed according to Diffley et al . , 1994, on 50 ml of culture (2xl0 7 cells/ml) . v) Northern and Western Blot Analysis The methods described by Cross and Tinkelenberg (1991) and Price et al . (1991) were used for RNA isolation and Northern blot analysis respectively. For Western blot analysis, protein extracts were prepared either as described in Surana et al . (1993) (Fig. 2C and 4B) , or by TCA precipitation (Foiani et al . , 1994) (Example 5B) . 50-150 mg of total extracts were transferred to Immobilon P membranes (Millipore) . HA tagged Cdc6 was detected with 12CA5 MAb (1:100) , while myc tagged Cdc6 was detected by MAb 9E10 (1:200) ; the signal was amplified as previously described (Piatti et al . , 1995) . Anti-Swi6 Ab were used at 1:100,000 and anti-Cdc28 Ab at 1:1000 dilution. Secondary antibodies were purchased from Amersham and proteins were detected by an enhanced chemiluminescence system according to the manifacturer. vi) Immunoprecipitation and kinase assays Protein extracts were prepared as in Schwob et al . , 1994. For Clb2 kinase assays, 100 mg of total proteins were immunoprecipitated using anti-Clb2 Ab (1:30, Amon et al . , 1992) . To assay Cdc6-associated kinase, HA3Cdc6 or Cdc6HA3 was immunoprecipitated with 12CA5 Ab (1:10) from the amounts of protein extracts indicated in figure legends. Histone HI kinase activity was measured as previously described (Schwob et al., 1994) . vii) Detection of the in vivo Association between ARS Sequences and Relevant Proteins The procedures are based on the methods described by Hecht, A. et al . , 1996; Strahl-Bolsinger et al . , 1997) with some modifications. Fifty milliliters of yeast cells (1.0-2.0 ' 10 7 cells/ml) were crosslinked with 1 % formaldehyde for 15 min at room temperature. After addition of 125 mM glycine and incubation for 5 min, cells were harvested and washed three times with Tris- buffered saline. The cell breakage was performed in the 400-800 ml lysis buffer (see the above references) with glassbeads. And the cell extracts were sonicated four times for 15 sec each using micro-ultrasonic cell disruptor (Kontes) with a model AS1 probe (chromatin is sheared into an average size of 500 bp) . Immunoprecipitation was performed with magnetic beads (2.0 ' 10 7 beads) which are coated with rat monoclonal anti-mouse immunoglobulin (Dynabeads M-450, Dynal) and incubated for 3-8 hours with 9E11 (anti-myc antibody) or 12CA5 (anti-HA antibody) monoclonal antibody beforehand according to manufacturer's protocol. It was confirmed that more than 90 % of relevant myc-tagged proteins are immunoprecipitated by this procedure without prior crosslinking and that 50-60 % are immunoprecipitated after crosslinking. Precipitates were washed and processed for DNA purification as described in the above references. PCRs were carried out in 50 ml volume with 1/30-1/10 or 1/10 (This amount was optimized to amplify PCR products without reaching a plateau phase under the PCR cycles described below) of the material immunoprecipitated by an anti-myc or anti-HA antibody respectively, or 1/6000 of DNA samples derived from whole cell extract with prior crosslinking. Tag polymerase (GIBCO/BRL) and the corresponding buffer system were used. PCR primers were designed, as 20 mers with apporoximately 50 % GC content, to amplify the genome sequences locating at 454.5, 458.5, 462.5 (including ARS1) and 466.5 kb from the left telomere of chromosome IV in sizes of 350, 310, 270 and 228 bp, respectively, for the ARS1- containing region; and designed to amplify the genome sequences locating at 30.5, 34.5, 37.5 and 39.5 (including ARS305) kb from the left telomere of chromosome III in sizes of 240, 270, 310 and 346 bp, respectively, for the ARS305-containing region. Four pairs of primers were used together in each PCR reaction. The final concentration of each primer, which was set up so as to amplify each fragment evenly when total genome was used as a template in any concentrations, was 300 nM except for the primers to amplify 350 bp or 270 bp in ARS1-containing region (450 nM or 180 nM was employed, respectively) . In PCR cycles, an initial denaturation of 3 min at 94°C was followed by 30 cycles with denaturation for 1 min at 94°C, annealing for 1 min at 53°C, polymerization for 2 min at 72°C, and a final extension for 7 min at 72°C. In order to avoid primer-dimer formations, "hot start" procedure was achieved by anti-Tag antibody (Tag start antibody, Clontech) according to the manufacturer's protocol. Thirty percent of PCR products were separated in 2.3% agarose gel and visualized with 0.2 mg/ml ethidium bromide. The gels were photographed using Gel Print 2000i (Biophotonics) . viii) Other Techniques Flow cytometric DNA quantitation was determined according to Epstein and Cross (1992) on a Becton- Dickinson FACScan. In si tu immunofluorescence and photomicroscopy were performed according to Nasmyth et al . (1990) . To detect immunostaining of mycl2Cdc6, 9E10 MAb was used at a 1:5 dilution and the signal was detected by indirect immunofluorescence using CY3- conjugated anti-mouse Ab (1:200, Sigma) . Example 1 Pre-RCs cannot be formed in G2/M cdclS GAL-ubiCDC6 (K5033) cells were arrested into either α factor or nocodazole after having been deprived of Cdc6 protein (see Materials and Methods) . During the arrests the synthesis of Cdc6 protein was re-induced by shift to galactose medium. The triangles in Fig. 1 indicate increasing concentrations of DNAse I. The position of the ORC-induced hypersensitive site is indicated as an asterisk. Example 2 Cells deprived of Cdc6 in late mitosis are unable to replicate in the next cell cycle but undergo reductional" anaphase cdcl5 (K1993) and cdcl5 GAL-ubiCDC6 cells (K5032) growing in YEPRG medium were arrested by 3 hours incubation at 37°C. Afterwards, cells were filtered, resuspended in YEPR at 37°C and after 30' (time=0) released into either YEPR (Fig. 2A and B) or YEPRG medium at 25°C (Fig. 2C) . In Fig. 2D) are shown, for the same experiment described in Fig. 2A) , B) and C) , the fractions of 2C nuclei (i.e. nuclei which have undergone DNA replication) calculated with the following equation : %2C = 100. (2.C4 + U - Ci)/ (U + 2.B), where Ci, C2, and C4 are the fractions of cells at each point with IC, 2C, or 4C DNA contents, and U and B the fractions of uni- and binucleated cells respectively (see Materials and Methods) . Example 3 Cells pass a "point of no return" at the time of S phase entry after which Cdc6 synthesis is unable to promote DNA replication A) cdclS (K1993) and cdc!5 GAL-ubi CDC6 (K5032) cells were treated as in Example 2. After release from the cdcl5 arrest in YEPR (t=0) , aliquots of the cdclΞ GAL- ubiCDCβ culture, withdrawn at 10' intervals (indicated on the right in Fig. 3A) , were supplemented with 2% galactose to re-induce Cdc6 synthesis. For each independent culture, cell samples were analysed by FACS every 30 minutes for 2 hours. The fractions of 2C nuclei at each time point has been calculated using the formula described in Example 2 (Fig. 2D) . The time indicated on the right of Fig. 3A represents the time at which each aliquot of culture has been transferred to galactose medium. B) The same experimental procedure used in A) has been applied to a GAL-HA3CDC6 strain (K5763) , with the exception that 0.1% galactose has been used to re- induce Cdc6 synthesis (Fig. 3B) . Under the same experimental conditions as in B) , expression of HA3Cdc6 at the different time points has been evaluated by Western analysis after 30' of induction. Swi6p was used as an internal loading control (Fig. 3C) . Example 4 Deletion of CLB5 and CLB6 causes a delay in S phase entry and a similar delay in the "point of no return" cdcl5 (K1993) and cdc!5 clb5 clbβ (K5027) cells growing in YEPD were arrested for 3.5 hours at 37°C and then released at 25°C. At different time points cell samples were withdrawn for FACS analysis and budding index (Fig. 4A) and to assay Clb2-dependent histone HI kinase activity (Fig. 4B) . C) cdclδ clb5 clbβ GAL-ubiCDCβ cells (K5231) were treated as described for K5032 in Example 3. After release from cdcl5 arrest into YEPR (t=0) , aliquots of K5321 culture, withdrawn at 10 minutes intervals, were supplemented with 2% galactose and incubated in YEPRG for 180 minutes. From each individual culture, cell samples were collected every 30 minutes for FACS analysis and the fractions of 2C nuclei have been evaluated using the equation as described in Example 2. The results are shown in Fig. 4C. Example 5 B-type cyclin dependent Cdc28 kinase associates with Cdc6p in vivo during S, G2 and M phases. A) Cell extracts from wild type cells containing an untagged (K699) or a HA3 tagged version of CDC6 ( CDC6HA3 , K4528) have been incubated in either the presence (+Ab) or in the absence (-Ab) of 12CA5 Ab. After immunoprecipitation, histone HI kinase has been assayed (Fig. 5A) . B) K4528 (wild type) , K5275 ( cdc28-13) , K5082 ( cdc34 -2) , K5272 i cdc7-l) , K5074 { cdc34 -2, sicl ) , K5279 ( cdcl5-2) cells, all containing the CDC6HA3 gene, have been grown in YEPD at 25°C and arrested for 3.5 hours at 37°C. K4528 cells have also been incubated in YEPD with 80 mM hydroxyurea (HU) at 25°C for 3.5 hours. Cell samples have been collected for Western blot with the 12CA5 Ab(lower panel of Fig. 5B) , kinase assays and FACS analysis (not shown) . C (loading control) is a protein cross-reacting with the 12CA5 Ab. For kinase assays, Cdc6p was immunoprecipitated with 12CA5 Ab from 0.5 mg of protein extracts and histone HI kinase activity was measured (Fig. 5B) . C) Cdc6-associated kinase is dependent on CDC28. Wild type (WT, K4527) and cdc28 -4 (K5472) cells, both containing 5 copies of GAL-HA3CDC6, were grown in YEPR and induced for 4 hours in YEPRG at 25°C; K5472 cells were also arrested for 3 hours in YEPR at 37°C or incubated for 1 hour in YEPRG at 25°C and then arrested for 3 hours at 37°C. For each condition 0.2 mg of protein extracts were immunoprecipitated with 12CA5 Ab and the associated kinase activity was measured (Fig. 5C. R: Raffinose. G: Galactose) . D) Cdc6-associated kinase is inhibited by p40 SIC1 . 1 mg of protein extract from K4527, grown in YEPR at 25°C or induced for 4 hours in YEPRG, was immunoprecipitated with 12CA5 Ab and before the last washing step split into four aliquots. Kinase activity was measured in the absence or in the presence of the indicated amounts of purified p40 SIC1 . R:Raffinose. G: Galactose. The addition of p40 SJC '- 1 causes the appearance of two additional signals probably due to phosphorylation of p40 SJ Cl ^ an aspecific kinase. E) Cdc28p co- immunoprecipitates with Cdc6p. Cdc6p was immunoprecipitated with 12CA5 Ab from protein extracts (5 mg) of K4527 cells grown at 25°C in YEPR and induced for 4 hours in YEPRG. Protein A-Sepharose beads were boiled in SDS buffer and analysed by Western blot using Cdc28 Ab (1:500) (Fig. 5D. R: Raffinose. G: Galactose) . Example 6 In vivo Associations of Orc2, Cdc6 and Mcm7 Proteins with ARSs These associations were detected by formaldehyde- crosslinking and a subsequent immunoprecipitation. (A) In vivo association of Orc2p with ARS1 and ARS305. PCR was performed with co-immunoprecipitated chromatin fragments (Fig. 7A, lane 1-4) , DNA samples from the whole cell extract (WCE) with prior crosslinking (lane 5, 8) or serial four-fold dilution thereof (lane 6, 7, 9, 10) for templates and four sets of primers to amplify three regions surrounding ARS1 and one including ARS1 (top) . Similar procedures were employed for ARS305 (bottom) . These DNA samples were prepared from the cells, which were asynchronously cultured in YEPR at 25°C, with (K6447, lane 2-4, 8-10) or without (K699, lane 1, 5-7) myc-tags for 0rc2. Immunoprecipitation procedures were performed with (lane 1-3) or without (lane 4) prior crosslinking, with (lane 1, 3, 4) or without (lane 2) anti-myc antibody. (B) In vivo association of Cdc6p and Mcm7p with ARS1 (Fig. 7B top) and ARS305 (bottom) . PCRs were carried out with co-immunoprecipitated chromatin fragments (lane 1-8) , DNA samples from the WCE with prior crosslinking (lane 9-12) . These samples were prepared from asynchronously growing cells without (K699, lane 1, 9) myc-tags or with them for Mcm7 (K6210, lane 2-4, 10), Cdc6 (K6302, lane 5-7, 11) or Csel (K6182, lane 8, 12) . Immunoprecipitation procedures were performed with (lane 1-3, 5, 6, 8) or without (lane 4, 7) prior crosslinking, with (lane 1, 3, 4, 6-8) or without (lane 2, 5) anti-myc antibody. (C) ARS1 mutations abolish in vivo association of Orc2p, Cdc6p and Mcm7p with ARS1 (Fig. 7C top) but not with ARS305 (bottom) . PCRs were carried out with co- immunoprecipitated chromatin fragments (lane 1-12) , DNA samples from the WCE with prior crosslinking (lane 13- 15) .These samples were prepared from asynchronous cells without (lane 1-3) myc-tags or with them fused to 0rc2 (lane 4-6) , Mcm7 (lane 7-9, 13-15) or Cdc6 (lane 10- 12) . These cells harbor wild type ARS1 (lane 1 K6653, lane 4 K6649, lane 7 plus 13 K6670, lane 10 K6675) , a mutation in element A (lane 2 K6638, lane 5 K6641, lane 8 plus 14 K6639, lane 11 K6673) or mutations in element Bl-3 (lane 3 K6667, lane 6 K6662, lane 9 plus 15 K6666, lane 12 K6672) within ARS1. PCRs with DNA from WCEs of strains expressing no myc-tag, Orc2-myc, Cdc6-myc gave the results similar to those in case of Mcm7-myc (data not shown) . Among ARS1 mutants and its wild type, there was no significant difference in DNA contents measured by FACS. Example 7 Cell Cycle-regulation of the Association of Cdc6p and Mcm7p with ARSs (A) In vivo association of Cdc6p with ARS1 in cell cycle-synchronized cells. Small GI cells of Cdcβ-myc homozygous diploid (K6691) were isolated by centrifugal elutriation and then released into YEPR at 25°C. Template DNAs for PCR were prepared from the formaldehyde crosslinked cells derived from the same volume of sample at each time point, after immunoprecipitation (0 - 200 min) or WCE (The results are shown in Fig. 8A. The most left lane shows a sample in 0 min) . DNAs from WCE at other time points show the almost the same results as that in 0 min (data not shown) . The percentage of budded cells and the cells with long spindle, and DNA contents measured by FACS are also shown. (B) In vivo association of Mcm7p with ARS1 (Fig. 8B top) and ARS305 (bottom) in cell cycle-synchronized cells. Small GI cells of Mcm7-myc homozygous diploid (K6465) were isolated by centrifugal elutriation and then released into YEPR at 25°C. Template DNAs for PCR were prepared in the same way as in (A) . The percentage of budded cells, the cells with long spindle and the cells showing nuclear accumulation of Mcm7p as well as DNA contents measured by FACS are also shown. Example 8 Association of Mcm7p with ARSs and Chromatin Depends on the Presence of Cdc6p Early GI cells of the GAL-uJbi-CDC6- Dcdcβ -MCM7 -myc homozygous diploid (K6484) were isolated by centrifugal elutriation after depleting Cdc6p by removal of galactose from the medium for 75 min. They were subsequently released into either YEPR (Gal-) or YEPRGal (Gal-f) medium at 25°C. In vivo association of Mcm7p with ARS1 (Fig. 9 top) and ARS305 (bottom) . Template DNAs for PCR were prepared in the same way as in Figure 8A at indicated time points. 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Claims: Claims 1. Compounds which interfere with the function of the cdc6 protein to form or maintain pre-replication complexes in an animal cell without impairing the cell's ability to activate cyclin dependent kinases that promote S phase and/or M phase, for use in a method of killing rapidly dividing cells, in particular tumour cells, by inhibiting DNA replication. 2. Compounds of claim 1 for use in cancer therapy. 3. A method of screening for compounds of claim 1 with the ability to interfere with DNA replication in an animal cell, wherein a test compound is applied to a substrate comprising the components required for DNA replication, and wherein the effect of the substance on the ability of the cdc6 protein to form or maintain pre-replication complexes is determined directly or indirectly. 4. The method of claim 3, wherein a substrate comprising a mixture of an egg extract, which is capable of DNA replication, and chromatin, is incubated with the test compound for a period of time sufficient for cdc6p to form pre-replication complexes, and wherein assembly of the Cdc6 protein onto chromatin is determined directly. 5. The method of claim 3, wherein a substrate comprising a mixture of human sperm chromatin or nuclei isolated from human GI cells, an extract prepared from human cells that express high levels of cdc6p, and a cyclin dependent kinase inhibitor, is incubated with the test compound for a period of time sufficient for cdc6p to form pre-replication complexes, and wherein assembly of the Cdc6 protein onto chromatin is determined directly. 6. The method of of claim 4 or 5, wherein assembly of Cdc6 protein onto chromatin is determined by a anti-cdc6 antibody. 7. The method of claim 6, wherein the anti-cdc6 antibody carries an fluorescent label. 8. The method of claim 3, wherein the substrate comprises rapidly dividing animal cells, in particular human cells derived from a tumour cell line, wherein the cells are incubated in the presence of the test substance for a period of time sufficient for cdc6p to form pre-replication complexes, and wherein the cells are lysed and assembly of Cdc6 protein onto chromatin is determined directly. 9. The method of claim 3, wherein the substrate comprises yeast cells that carry an ore mutation and an inducible plasmid containing the human CDC6 sequence such that they overexpress cdc6p, wherein the cells are grown and incubated with the test substance, and wherein the effect of the compound to inhibit cdc6p function is determined indirectly by determining survival of the cells. 10. A method of any one of claims 3 to 9, wherein the ability of the compound not to impair activation of cyclin dependent kinases that promote S phase and/or M phase is determined by measuring kinase activity and/or cyclin accumulation. 11. Pharmaceutical composition comprising as an active ingredient a compound as defined in claim 1.

Inventors: DIFFLEY JOHN, NASMYTH KIM, PIATTI SIMONETTA, DIFFLEY, JOHN, NASMYTH, KIM, PIATTI, SIMONETTA
Applicants: BOEHRINGER INGELHEIM INT, DIFFLEY JOHN, NASMYTH KIM, PIATTI SIMONETTA, BOEHRINGER INGELHEIM INTERNATIONAL GMBH, DIFFLEY, JOHN, NASMYTH, KIM, PIATTI, SIMONETTA
Application year: 1997
Main classification: A61K49
IPC codes: A61K31, A61K49, C12N5, C12N15