Fanconi anemia (FA) is a human autosomal disorder characterized by cancer 
susceptibility and cellular sensitivity to DNA crosslinking agents such as 
mitomycin C and diepoxybutane. Six FA genes have been cloned including a gene 
designated XRCC9 (for X-ray Repair Cross Complementing), isolated using a 
mitomycin C-hypersensitive Chinese hamster cell mutant termed UV40, and 
subsequently found to be identical to FANCG. A nuclear complex containing the 
FANCA, FANCC, FANCE, FANCF and FANCG proteins is needed for the activation of a 
sixth FA protein FANCD2. When monoubiquitinated, the FANCD2 protein co-localizes 
with the breast cancer susceptibility protein BRCA1 in DNA damage induced foci. 
In this study, we have assigned NM3, a nitrogen mustard-hypersensitive Chinese 
hamster mutant to the same genetic complementation group as UV40. NM3, like 
human FA cell lines (but unlike UV40) exhibits a normal spontaneous level of 
sister chromatid exchange. We show that both NM3 and UV40 are also 
hypersensitive to other DNA crosslinking agents (including diepoxybutane and 
chlorambucil) and to non-crosslinking DNA damaging agents (including bleomycin, 
streptonigrin and EMS), and that all these sensitivities are all corrected upon 
transfection of the human FANCG/XRCC9 cDNA. Using immunoblotting, NM3 and UV40 
were found not to express the active monoubiquitinated isoform of the FANCD2 
protein, although expression of the FANCD-L isoform was restored in the FANCG 
cDNA transformants, correlating with the correction of mutagen-sensitivity. 
These data indicate that cellular resistance to these DNA damaging agents 
requires FANCG and that the FA gene pathway, via its activation of FANCD2 and 
that protein's subsequent interaction with BRCA1, is involved in maintaining 
genomic stability in response not only to DNA interstrand crosslinks but also a 
range of other DNA damages including DNA strand breaks. NM3 and other "FA-like" 
Chinese hamster mutants should provide an important resource for the study of 
these processes in mammalian cells.