S-phase arrest by ara-C: abrogation by UCN-01. Exponentially growing ML-1 cells were incubated with 50 nM ara-C for 24 hours (○), at which time the culture was split and UCN-01 (50 nM) was added to one portion (•). Cells were sampled from each culture at the indicated times. DNA content (A, S phase; B, G₂/M; C, G₁) and TUNEL reactivity (D) were analyzed by flow cytometry. Symbols represent mean ± SD.