[Federal Register Volume 62, Number 225 (Friday, November 21, 1997)]
[Notices]
[Pages 62472-62475]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-30706]



[[Page 62471]]

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Part V





Department of Health and Human Services





_______________________________________________________________________



Food and Drug Administration



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International Conference on Harmonisation; Guidance on Genotoxicity: A 
Standard Battery for Genotoxicity Testing of Pharmaceuticals; 
Availability; Notice

Federal Register / Vol. 62, No. 225 / Friday, November 21, 1997 / 
Notices

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DEPARTMENT OF HEALTH AND HUMAN SERVICES

Food and Drug Administration
[Docket No. 97D-0112]


International Conference on Harmonisation; Guidance on 
Genotoxicity: A Standard Battery for Genotoxicity Testing of 
Pharmaceuticals; Availability

AGENCY: Food and Drug Administration, HHS.

ACTION: Notice.

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SUMMARY: The Food and Drug Administration (FDA) is publishing a 
guidance entitled ``S2B Genotoxicity: A Standard Battery for 
Genotoxicity Testing of Pharmaceuticals.'' The guidance was prepared 
under the auspices of the International Conference on Harmonisation of 
Technical Requirements for Registration of Pharmaceuticals for Human 
Use (ICH). The guidance identifies a standard set of genotoxicity tests 
that should be conducted for pharmaceutical registration and recommends 
the extent of confirmatory experimentation in in vitro genotoxicity 
tests in the standard battery. The guidance complements the ICH 
guidance ``Guidance on Specific Aspects of Regulatory Genotoxicity 
Tests for Pharmaceuticals'' (S2A).

DATES: Effective November 21, 1997. Submit written comments at any 
time.

ADDRESSES: Submit written comments on the guidance to the Dockets 
Management Branch (HFA-305), Food and Drug Administration, 12420 
Parklawn Dr., rm. 1-23, Rockville, MD 20857. Copies of the guidance are 
available from the Drug Information Branch (HFD-210), Center for Drug 
Evaluation and Research, Food and Drug Administration, 5600 Fishers 
Lane, Rockville, MD 20857, 301-827-4573.

FOR FURTHER INFORMATION CONTACT:
    Regarding the guidance: Robert E. Osterberg, Center for Drug 
Evaluation and Research (HFD-520), Food and Drug Administration, 9201 
Corporate Blvd., Rockville, MD 20850, 301-827-2123.
    Regarding ICH: Janet J. Showalter, Office of Health Affairs (HFY-
20), Food and Drug Administration, 5600 Fishers Lane, Rockville, MD 
20857, 301-827-0864.

SUPPLEMENTARY INFORMATION: In recent years many important initiatives 
have been undertaken by regulatory authorities and industry 
associations to promote international harmonization of regulatory 
requirements. FDA has participated in many meetings designed to enhance 
harmonization and is committed to seeking scientifically based 
harmonized technical procedures for pharmaceutical development. One of 
the goals of harmonization is to identify and then reduce differences 
in technical requirements for drug development among regulatory 
agencies.
    ICH was organized to provide an opportunity for tripartite 
harmonization initiatives to be developed with input from both 
regulatory and industry representatives. FDA also seeks input from 
consumer representatives and others. ICH is concerned with 
harmonization of technical requirements for the registration of 
pharmaceutical products among three regions: The European Union, Japan, 
and the United States. The six ICH sponsors are the European 
Commission, the European Federation of Pharmaceutical Industries 
Associations, the Japanese Ministry of Health and Welfare, the Japanese 
Pharmaceutical Manufacturers Association, the Centers for Drug 
Evaluation and Research and Biologics Evaluation and Research, FDA, and 
the Pharmaceutical Research and Manufacturers of America. The ICH 
Secretariat, which coordinates the preparation of documentation, is 
provided by the International Federation of Pharmaceutical 
Manufacturers Associations (IFPMA).
    The ICH Steering Committee includes representatives from each of 
the ICH sponsors and the IFPMA, as well as observers from the World 
Health Organization, the Canadian Health Protection Branch, and the 
European Free Trade Area.
    In the Federal Register of April 3, 1997 (62 FR 16026), FDA 
published a draft tripartite guideline entitled ``Genotoxicity: A 
Standard Battery for Genotoxicity Testing of Pharmaceuticals'' (S2B). 
The notice gave interested persons an opportunity to submit comments by 
June 2, 1997.
    After consideration of the comments received and revisions to the 
guidance, a final draft of the guidance was submitted to the ICH 
Steering Committee and endorsed by the three participating regulatory 
agencies on July 16, 1997.
    In accordance with FDA's Good Guidance Practices (62 FR 8961, 
February 27, 1997), this document has been designated a guidance, 
rather than a guideline.
    Genotoxicity tests are in vitro and in vivo tests designed to 
detect compounds that induce genetic damage directly or indirectly by 
various mechanisms. Compounds that are positive in tests that detect 
such damage have the potential to be human carcinogens and/or mutagens, 
i.e., may induce cancer and/or heritable defects. The guidance 
addresses two areas of genotoxicity testing for pharmaceuticals: (1) 
Identification of a standard set of tests that should be conducted for 
registration and (2) the extent of confirmatory experimentation in in 
vitro genotoxicity tests in the standard battery. The guidance is 
intended to be used together with the ICH S2A guidance entitled 
``Guidance on Specific Aspects of Regulatory Genotoxicity Tests for 
Pharmaceuticals'' (61 FR 18198, April 24, 1996) as ICH guidance 
principles for testing pharmaceuticals for potential genotoxicity.
    This guidance represents the agency's current thinking on a 
recommended standard battery for genotoxicity testing of a 
pharmaceutical. It does not create or confer any rights for or on any 
person and does not operate to bind FDA or the public. An alternative 
approach may be used if such approach satisfies the requirements of the 
applicable statute, regulations, or both.
    As with all of FDA's guidances, the public is encouraged to submit 
written comments with new data or other new information pertinent to 
this guidance. The comments in the docket will be periodically 
reviewed, and, where appropriate, the guidance will be amended. The 
public will be notified of any such amendments through a notice in the 
Federal Register.
    Interested persons may, at any time, submit written comments on the 
guidance to the Dockets Management Branch (address above). Two copies 
of any comments are to be submitted, except that individuals may submit 
one copy. Comments are to be identified with the docket number found in 
brackets in the heading of this document. The guidance and received 
comments may be seen in the office above between 9 a.m. and 4 p.m., 
Monday through Friday. An electronic version of this guidance is 
available on the Internet (http://www.fda.gov/cder/guidance.htm).
    The text of the guidance follows:

S2B Genotoxicity: A Standard Battery for Genotoxicity Testing of 
Pharmaceuticals\1\
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    \1\ This guidance represents the agency's current thinking on a 
recommended standard battery for genotoxicity testing of a 
pharmaceutical. It does not create or confer any rights for or on 
any person and does not operate to bind FDA or the public. An 
alternative approach may be used if such approach satisfies the 
requirements of the applicable statute, regulations, or both.
---------------------------------------------------------------------------

1. Introduction

    Two fundamental areas in which harmonization of genotoxicity 
testing for

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pharmaceuticals is considered necessary are the scope of this 
guidance: (I) Identification of a standard set of tests that should 
be conducted for registration. (II) The extent of confirmatory 
experimentation in in vitro genotoxicity tests in the standard 
battery. Further issues that were considered necessary for 
harmonization can be found in the ICH guidance S2A ``Guidance on 
Specific Aspects of Regulatory Genotoxicity Tests for 
Pharmaceuticals.'' The two ICH guidances on genotoxicity complement 
each other and therefore should be used together as ICH guidance 
principles for testing of a pharmaceutical for potential 
genotoxicity.

2. General Purpose of Genotoxicity Testing

    Genotoxicity tests can be defined as in vitro and in vivo tests 
designed to detect compounds that induce genetic damage directly or 
indirectly by various mechanisms. These tests should enable a hazard 
identification with respect to damage to DNA and its fixation. 
Fixation of damage to DNA in the form of gene mutations, larger 
scale chromosomal damage, recombination and numerical chromosome 
changes is generally considered to be essential for heritable 
effects and in the multistep process of malignancy, a complex 
process in which genetic changes may play only a part. Compounds 
which are positive in tests that detect such kinds of damage have 
the potential to be human carcinogens and/or mutagens, i.e., may 
induce cancer and/or heritable defects. Because the relationship 
between exposure to particular chemicals and carcinogenesis is 
established for man, while a similar relationship has been difficult 
to prove for heritable diseases, genotoxicity tests have been used 
mainly for the prediction of carcinogenicity. Nevertheless, because 
germ line mutations are clearly associated with human disease, the 
suspicion that a compound may induce heritable effects is considered 
to be just as serious as the suspicion that a compound may induce 
cancer. In addition, the outcome of such tests may be valuable for 
the interpretation of carcinogenicity studies.

3. The Standard Test Battery for Genotoxicity

    Registration of pharmaceuticals requires a comprehensive 
assessment of their genotoxic potential. It is clear that no single 
test is capable of detecting all relevant genotoxic agents. 
Therefore, the usual approach should be to carry out a battery of in 
vitro and in vivo tests for genotoxicity. Such tests are 
complementary rather than representing different levels of 
hierarchy.
    The general features of a standard test battery can be outlined 
as follows:
    (i) It is appropriate to assess genotoxicity in a bacterial 
reverse mutation test. This test has been shown to detect relevant 
genetic changes and the majority of genotoxic rodent carcinogens.
    (ii) DNA damage considered to be relevant for mammalian cells 
and not adequately measured in bacteria should be evaluated in 
mammalian cells. Several mammalian cell systems are in use: Systems 
that detect gross chromosomal damage (in vitro tests for structural 
and numerical chromosomal aberrations), systems that detect 
primarily gene mutations (see Note 1), and a system that detects 
gene mutations and clastogenic effects (mouse lymphoma tk assay) 
(see Note 2). The information given in Notes 3 and 4 demonstrates 
that with appropriate test protocols (see section 5 of this 
document) the various in vitro tests for chromosomal damage and the 
mouse lymphoma tk assay yield results with a high level of 
congruence for compounds that are regarded as genotoxic but yield 
negative results in the bacterial reverse mutation assay. Therefore, 
these systems are currently considered interchangeable when used 
together with other genotoxicity tests in a standard battery for 
genotoxicity testing of pharmaceuticals, if these test protocols are 
used.
    (iii) An in vivo test for genetic damage should usually be a 
part of the test battery to provide a test model in which additional 
relevant factors (absorption, distribution, metabolism, excretion) 
that may influence the genotoxic activity of a compound are 
included. As a result, in vivo tests permit the detection of some 
additional genotoxic agents (see Note 5). An in vivo test for 
chromosomal damage in rodent hematopoietic cells fulfills this need. 
This in vivo test for chromosomal damage in rodents could be either 
an analysis of chromosomal aberrations in bone marrow cells or an 
analysis of micronuclei in bone marrow or peripheral blood 
erythrocytes.
    The following standard test battery is recommended based upon 
the considerations mentioned above:
    (i) A test for gene mutation in bacteria.
    (ii) An in vitro test with cytogenetic evaluation of chromosomal 
damage with mammalian cells or an in vitro mouse lymphoma tk assay.
    (iii) An in vivo test for chromosomal damage using rodent 
hematopoietic cells.
For compounds giving negative results, the completion of this 3-test 
battery, performed and evaluated in accordance with current 
recommendations, will usually provide a sufficient level of safety 
to demonstrate the absence of genotoxic activity (see Note 6). 
Compounds giving positive results in the standard test battery may, 
depending on their therapeutic use, need to be tested more 
extensively (see ICH S2A ``Guidance on Specific Aspects of 
Regulatory Genotoxicity Tests for Pharmaceuticals'').
    The suggested standard set of tests does not imply that other 
genotoxicity tests are generally considered inadequate or 
inappropriate (e.g., tests for measurement of DNA adducts, DNA 
strand breaks, DNA repair or recombination). Such tests serve as 
options in addition to the standard battery for further 
investigation of genotoxicity test results obtained in the standard 
battery. Furthermore, molecular techniques to study mechanisms of 
genotoxicity in the standard battery systems may be useful for risk 
assessment. Only under extreme conditions in which one or more tests 
comprising the standard battery cannot be employed for technical 
reasons, alternative validated tests can serve as substitutes. For 
this to occur, sufficient scientific justification should be 
provided to support the argument that a given standard battery test 
is not appropriate.
    The standard battery does not include an independent test 
designed specifically to test for aneuploidy. However, information 
on this type of damage may be derived from the tests for chromosomal 
damage in vitro and in vivo. Elements of the standard protocols that 
provide such information are elevations in the mitotic index, 
polyploidy induction and micronucleus evaluation. There is also 
limited experimental evidence that aneuploidy inducers can be 
detected in the mouse lymphoma tk assay (see Note 4). In such cases, 
further testing may be needed.

4. Modifications of the 3-Test Battery

    The following sections give situations where the standard 3-test 
battery may need modification.

4.1 Limitations to the Use of Bacterial Test Organisms

    There are circumstances where the performance of the bacterial 
reverse mutation test does not provide appropriate or sufficient 
information for the assessment of genotoxicity. This may be the case 
for compounds that are excessively toxic to bacteria (e.g., some 
antibiotics) and compounds thought or known to interfere with the 
mammalian cell replication system (e.g., topoisomerase inhibitors, 
nucleoside analogues, or inhibitors of DNA metabolism). For these 
cases, usually two in vitro mammalian cell tests should be performed 
using two different cell types and of two different endpoints (gene 
mutation (see Note 1) and chromosomal damage). Nevertheless, it is 
still important to perform the bacterial reverse mutation test (see 
Note 7); either a full test or a limited (range-finding) test (see 
section 5 of this document) may be appropriate.

4.2 Compounds Bearing Structural Alerts for Genotoxic Activity

    Structurally alerting compounds (see Note 8) are usually 
detectable in the standard 3-test battery. However, compounds 
bearing structural alerts that have given negative results in the 
standard 3-test battery may necessitate limited additional testing. 
The choice of additional test(s) or protocol modification(s) depends 
on the chemical nature, the known reactivity, and metabolism data on 
the structurally alerting compound under question (see Note 9 and 
ICH S2A ``Guidance on Specific Aspects of Regulatory Genotoxicity 
Tests for Pharmaceuticals'').

4.3 Limitations to the Use of Standard In Vivo Tests

    There are compounds for which standard in vivo tests do not 
provide additional useful information. These include compounds for 
which data from studies on toxicokinetics or pharmacokinetics 
indicate that they are not systemically absorbed and therefore are 
not available for the target tissues in standard in vivo 
genotoxicity tests. Examples of such compounds are some radioimaging 
agents, aluminum-based antacids, and some dermally applied 
pharmaceuticals. In cases where a modification of the route of 
administration does not provide sufficient target tissue exposure, 
it may be appropriate to base the evaluation only on in vitro 
testing.

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4.4 Additional Genotoxicity Testing in Relation to the 
Carcinogenicity Bioassay

4.4.1 Evidence for Tumor Response

    Additional genotoxicity testing in appropriate models may be 
conducted for compounds that were negative in the standard 3-test 
battery but which have shown effects in carcinogenicity bioassay(s) 
with no clear evidence for a nongenotoxic mechanism. To help 
understand the mechanism of action, additional testing can include 
modified conditions for metabolic activation in in vitro tests or 
can include in vivo tests measuring genetic damage in target organs 
of tumor induction (e.g., liver UDS test, 32P-postlabeling, mutation 
induction in transgenes, molecular characterization of genetic 
changes in tumor-related genes).

4.4.2 Structurally Unique Chemical Classes

    On rare occasions, a completely novel compound in a unique 
structural chemical class will be introduced as a pharmaceutical. 
When such a compound will not be tested in chronic rodent 
carcinogenicity bioassays, further genotoxicity evaluation may be 
invoked.

5. Standard Procedures for In Vitro Tests

    Reproducibility of experimental results is an essential 
component of research involving novel methods or unexpected 
findings; however, the routine testing of chemicals with standard, 
widely used genotoxicity tests need not always be completely 
replicated. These tests are sufficiently well characterized and have 
sufficient internal controls that repetition can usually be avoided 
if protocols with built-in confirmatory elements, such as those 
outlined below, are used.
    For both bacterial and mammalian cell gene mutation tests, the 
results of a range-finding test can be used to guide the selection 
of concentrations to be used in the definitive mutagenicity test. By 
these means, a range-finding test may supply sufficient data to 
provide reassurance that the reported result is the correct one. In 
bacterial mutagenicity tests, preliminary range-finding tests 
performed on all bacterial strains, with and without metabolic 
activation, with appropriate positive and negative controls, and 
with quantification of mutants, may be considered a sufficient 
replication of a subsequent complete test. Similarly, a range-
finding test may also be a satisfactory substitute for a complete 
repeat of a test in gene mutation tests with mammalian cells other 
than the mouse lymphoma tk assay (see below) if the range-finding 
test is performed with and without metabolic activation, with 
appropriate positive and negative controls, and with quantification 
of mutants (see Note 10).
    For the cytogenetic evaluation of chromosomal damage in vitro, 
the test protocol includes the conduct of tests with and without 
metabolic activation, with appropriate positive and negative 
controls, where the exposure to the test articles is 3 to 6 hours 
and a sampling time of approximately 1.5 normal cell cycles from the 
beginning of the treatment. A continuous treatment without metabolic 
activation up to the sampling time of approximately 1.5 normal cell 
cycles is needed in case of a negative result for the short 
treatment period without metabolic activation. Certain chemicals may 
be more readily detected by longer treatment or delayed sampling 
times, e.g., some nucleoside analogues or some nitrosamines. 
Negative results in the presence of a metabolic activation system 
may need confirmation on a case-by-case basis (see Note 11). In any 
case, information on the ploidy status should be obtained by 
recording the incidence of polyploid cells as a percentage of the 
number of metaphase cells. An elevated mitotic index or an increased 
incidence of polyploid cells may give an indication of the potential 
of a compound to induce aneuploidy. In such cases, further testing 
may be needed.
    For the mouse lymphoma tk assay, the test protocol includes the 
conduct of tests with and without metabolic activation, with 
appropriate positive and negative controls, where the exposure to 
the test articles is 3 to 4 hours. A continuous treatment without 
metabolic activation for approximately 24 hours is needed in case of 
a negative result for the short treatment without metabolic 
activation (see Note 4). Negative results in the presence of a 
metabolic activation system may need confirmation on a case by case 
basis (see Note 11). In any case, an acceptable mouse lymphoma tk 
assay includes (i) the incorporation of positive controls, which 
induces mainly small colonies and (ii) colony sizing for positive 
controls, solvent controls, and at least one positive test compound 
dose (should any exist), including the culture that gave the 
greatest mutant frequency.
    Following such testing, further confirmatory testing in the case 
of clearly negative or positive test results is not usually needed.
    Ideally, it should be possible to declare test results as 
clearly negative or clearly positive. However, test results 
sometimes do not fit the predetermined criteria for a positive or 
negative call and therefore are declared ``equivocal.'' The 
application of statistical methods aids in data interpretation, 
however, adequate biological interpretation is of critical 
importance. Nonetheless, further testing is usually indicated for 
equivocal results.

6. Notes

    (1) Test approaches currently accepted for the assessment of 
mammalian cell gene mutation involve the tk locus using mouse 
lymphoma L5178Y cells or human lymphoblastoid TK6 cells, the hprt 
locus using CHO cells, V79 cells, or L5178Y cells, or the gpt locus 
using AS52 cells.
    (2) The molecular dissection of mutants induced at the tk locus 
shows a broad range of genetic events including point mutations, 
deletions, translocations, recombinations, etc. Small colony mutants 
have been shown to predominantly lack the tk<INF>b</INF> allele as a 
consequence of structural or numerical alterations or 
recombinational events. There is some evidence that other loci, such 
as hprt or gpt are also sensitive to large deletion events. However, 
due to the X-chromosomal origin of the hprt gene which is probably 
flanked by essential genes, large scale deletion events or numerical 
alterations often do not give rise to mutant colonies, thus limiting 
the sensitivity of this genetic locus relative to the tk locus for 
the detection of a wide range of genetic changes.
    (3) With respect to the cytogenetic evaluation of chromosomal 
damage, it is not uncommon for the systems currently in use, i.e., 
several systems with permanent mammalian cells in culture and human 
lymphocytes either isolated or in whole blood, to give different 
results for the same test compound. However, there is evidence that 
some of the differences observed have been due to protocol 
differences. This may be minimized by using the procedures described 
in section 5 of this document.
    For the great majority of presumptive genotoxic compounds that 
were negative in a bacterial reverse mutation assay, the data on 
chromosomal damage in vitro and mouse lymphoma tk results are in 
agreement. Several reliable studies indicate that the mouse lymphoma 
tk assay is able to detect compounds that induce structural and 
numerical chromosomal damage. For safety testing of pharmaceuticals, 
the mouse lymphoma tk assay is considered an acceptable alternative 
to the direct analysis of chromosomal damage in vitro. Although 
colony sizing is an essential element of the mouse lymphoma tk assay 
test protocol, it gives only limited information on the type of 
damage induced in mutant colonies. Further mechanistic 
investigations may be used to assess the nature of cytogenetic 
changes induced by clastogens and aneuploidy inducers in the mouse 
lymphoma tk assay. Such information could be provided by studies to 
demonstrate the loss of the tk gene or the loss of the chromosome 
carrying the tk gene.
    (4) The detection of a number of different nucleoside analogues 
and base analogues is enhanced for the mouse lymphoma tk assay when 
the treatment protocol for both agar and microtitre methods includes 
a 24-hour treatment regimen in the absence of an exogenous metabolic 
activation system. Similarly, the detection of aneuploidy inducers 
is enhanced if a 24-hour treatment regimen is used with the 
microtitre method. Currently, there is no evidence to support this 
conclusion for the soft agar method. The specificity of the test 
protocol, i.e., to obtain correct test results for presumptive 
nongenotoxic compounds, does not change significantly using a 24-
hour treatment in the microtitre method. For the soft agar method, 
there appears to be a reduction in specificity under the same 
treatment regimen. Based on this information, the microtitre method 
is recommended for use in the standard battery.
    (5) There are a small but significant number of genotoxic 
carcinogens that are reliably detected by the bone marrow tests for 
chromosomal damage that have yielded negative/weak/conflicting 
results in the pairs of in vitro tests outlined in the standard 
battery options, e.g., bacterial reverse mutation plus one of a 
selection of possible tests with cytogenetic evaluation of 
chromosomal damage or bacterial mutation plus the mouse lymphoma tk 
assay. Carcinogens such as procarbazine, hydroquinone, urethane and 
benzene fall into this category.
    (6) The continuing evolution of short-term tests and test 
methodologies will afford new,

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more sensitive, more practical, more expeditious, and more 
economical techniques for detection of genotoxic compounds. Some of 
these may ultimately replace the genotoxicity tests used for 
regulatory purposes. Among the more promising tests, the in vitro 
micronucleus test appears to offer potential for screening purposes.
    (7) Some antibacterial agents, albeit highly toxic to the tester 
strains, are detected as genotoxic at very low, sublethal 
concentrations in the bacterial reverse mutation test (e.g., 
nitrofuran antibiotics).
    (8) Certain structurally alerting molecular entities are 
recognized as being causally related to the carcinogenic and/or 
mutagenic potential of chemicals. Examples of structural alerts 
include alkylating electrophilic centers, unstable epoxides, 
aromatic amines, azo-structures, N-nitroso-groups, aromatic nitro-
groups.
    (9) For some classes of compounds with specific structural 
alerts, it is established that specific protocol modifications/
additional tests are necessary for optimum detection of genotoxicity 
(e.g., molecules containing an azo-group, glycosides, compounds such 
as nitroimidazoles requiring nitroreduction for activation, 
compounds such as phenacetin requiring another rodent S9 for 
metabolic activation). The additional testing needed when the chosen 
3-test battery yields negative results for a structurally alerting 
test compound could consist of such modifications.
    (10) The dose range-finding study should: (i) Give information 
on the shape of the toxicity dose-response curve if the test 
compound exhibits toxicity, (ii) include highly toxic 
concentrations, and (iii) include quantification of mutants in the 
cytotoxic range. If a compound is not toxic, then mutants should 
nevertheless be quantified.
    (11) A repetition of a test using the identical source and 
concentration of the metabolic activation system is usually not 
necessary. A modification of the metabolic activation system may be 
indicated for certain chemical classes where knowledge is available 
on specific requirements of metabolism. This would usually invoke 
the use of an external metabolizing system which is known to be 
competent for the metabolism/activation of the class of compound 
under test.

7. Glossary

    Cytogenetic evaluation: Chromosome structure analysis in mitosis 
or meiosis by light microscopy.
    DNA adduct: (Covalent) binding of chemicals to DNA.
    DNA repair: Reconstitution of damaged DNA sequence.
    DNA strand breaks: Single or double strand scissions in the DNA.
    Numerical chromosome changes: Chromosome numbers different from 
the original haploid or diploid set of chromosomes; for cell lines, 
chromosome numbers different from the modal chromosome set.
    Recombination: Breakage and balanced or unbalanced rejoining of 
DNA.
    Transgene: An exogenous or foreign gene inserted into the host 
genome, into either somatic cells or germ line cells.

    Dated: November 15, 1997.
William K. Hubbard,
Associate Commissioner for Policy Coordination.
[FR Doc. 97-30706 Filed 11-20-97; 8:45 am]
BILLING CODE 4160-01-F