Ž . Mutation Research 436 1999 11–19 Minireview Spontaneous mutations in the Big Blue w transgenic system are primarily mouse derived Kathleen A. Hill a , Victoria L. Buettner a , Barry W. Glickman b , Steve S. Sommer a, ) a Department of Molecular Genetics, Beckman Research Institute r City of Hope National Medical Center, 1500 East Duarte Road, Duarte, CA 91010-3000, USA b Center for EnÕironmental Health and The Department of Biology, UniÕersity of Victoria, Victoria, BC, Canada Received 21 July 1998; revised 29 October 1998; accepted 29 October 1998 Abstract The Big Blue w transgenic mouse mutation detection system provides a powerful approach for measuring spontaneous and induced mutations in vivo. The observed mutations may contain a fraction of ex vivo or prokaryotic mutational events. Indeed, a modified, selectable form of the Big Blue w assay seem to generate artifactual mutants under certain circumstances. Ž . Herein we review the evidence that circular mutants i.e., the plaque circumference is at least 50% blue collected in the standard Big Blue w assay are derived primarily from the mouse. The most direct evidence is the similarity in the types of mutations found in jackpot and nonjackpot mutations. In addition, about half of the spontaneous mutations in the lacI transgene are transitions and transversions at CpG dinucleotides, a mammalian-specific feature. The mutation pattern observed at lacI is consistent with AT mutation pressure operating in a GC rich DNA and approaches that reported for observed germline human factor IX mutations. Furthermore, the spontaneous mutation pattern of circular Big Blue w mutants Ž differs significantly from that of an endogenous lacI gene in E. coli. Pinpoint mutants a dot of blue color peripherally . located in a wild type plaque , which a priori were not expected to be mouse-derived, have a mutation pattern consistent with the mutation pattern of an endogenous E. coli lacI gene. Analysis of induced mutagenesis studies reveals mutation frequencies and patterns for the Big Blue w circular mutants which are comparable to endogenous genes. In reconstruction experiments, blue plaques derived from a superinfection with wild type and mutant phage produced approximately 50% blue and 50% clear plaques on replating. This phenomenon has not been seen when plaques derived from mouse were replated in the Big Blue w assay. Collectively, the evidence strongly supports a murine origin for circular mutants recovered in the standard Big Blue w assay. Validation of current assays is an essential step in determining the frequency and pattern of spontaneous murine-specific mutations. Defining this benchmark will be helpful in evaluating the next generation of transgenic mutation detection systems. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Big Blue w ; lacI transgene; Validation; DNA sequencing; Mutation pattern; Mutation signature; Jackpot mutation Ž . w x Abbreviations: Ethylnitrosourea ENU ; Benzo a pyrene Ž wx . B a P ) Corresponding author. Tel.: q1-626-930-5497; Fax: q1-626- 301-8142; E-mail: ssommer@coh.org 1. Transgenic mouse mutation detection assays Transgenic mouse mutation detection systems al- low the in vivo determination of the frequency and pattern of spontaneous or induced mutations in any 1383-5742r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S1383-5742 98 00024-6