Caffeine inhibits tachpyridine-induced G₂ arrest and activation of tachpyridine-induced CHK1 but not CHK2. (A) HeLa cells were either not pretreated or pretreated for 12 hours with 5 mM caffeine, followed by 18 hours of treatment with 7.5 μM tachpyridine, and analyzed by flow cytometry. (B) HeLa cells were either not pretreated or pretreated for 12 hours with 5 mM caffeine, followed by treatment with 7.5 μM tachpyridine over a time course of 3 to 24 hours, and analyzed by Western blotting (U indicates untreated control at 24 hours; caff, cells treated with caffeine alone for 24 hours).