In Fanconi anemia (FA) cells the duration of the G2phase of the cell cycle prolonged. Such a slowing of the G2phase can be induced in normal cells by irradiation with γ rays during S phase, which also further increases the duration of G2in FA cells. The addition of caffeine during the last 7h of culture shortens the G2phase in both nonirradiated and irradiated FA cells. In nonirradiated normal cells it may have no effect or may increase G2phase duration, but in irradiated normal reduces the slowing of G2induced by the radiation. This suggests that FA cells recognize and repair preexisting DNA lesions during G2phase and that caffeine inhibits this process. The principal anomaly in FA may be a deficient repair during S phase, as manifest in the prolonged postreplication repair period during G2phase required to repair the larger number of lesions passing through S phase
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