FA proteins localize to chromatin predominately after MMC treatment and during S phase. HeLa cells were synchronized by double thymidine block and collected. G₁-S cells were not released but were treated with 0.1 μM MMC for 4 hours. S-phase cells were prepared by simultaneous release for 4 hours in regular media and treated with 0.1 μM MMC for 4 hours. Chromatin was prepared and run on SDS-PAGE. After transfer, the membrane was blotted with anti-FANCA, anti-FANCG, and anti–topoisomerase II antisera. Neg indicates negative mutant EUFA143 FA-G cells that express no FANCG and very little FANCA. (B) 100 000 cells from the indicated treatment group were counted on FACScan at each time point.