UoC-M1 is sensitive to chromosomal breakage and radial formation following MMC treatment. Arrows indicate representative chromosomal anomalies in UoC-M1: (A) single chromatid break and (B) complex chromosomal rearrangement with at least 2 chromosomal breaks and one end-to-end fusion (radial). Such findings in clastogen-treated cells represent the clinical diagnostic laboratory standard for Fanconi anemia. UoC-M1 also demonstrated occasional, low-level chromosomal breakage without clastogen treatment, although most of the untreated cells had no evidence of instability. No radial forms or cells with multiple chromosomal breaks were ever noted in untreated specimens (of more than 500 total metaphases analyzed), although cells with single breaks were occasionally noted (maximum of 20% in an untreated specimen that when clastogen treated in a simultaneously prepared, parallel flask demonstrated 26% breakage with up to 3 breaks per cell and 38% radial formation; data not shown).