A cell kinetic method for the mitotic selection of treated G2 cells.

G2 cells treated with 150 rad X-radiation were isolated from a monolayer culture of exponentially growing Chinese hamster ovary (CHO) cells by a combination of 125Iododeoxyuridine ([125I]UdR)-induced blockade of S-phase cell progression, treatment and mitotic selection (125I-TMS technique). Once the rate at which cells were selected from a small window in mitosis was established (Schneiderman et al., 1972), the cells were exposed to 10 microCi/ml, carrier-free [125I]UdR for 10 min immediately before treatment with 150 rads X-radiation. After X-irradiation the cells located later in the cell cycle than the X-ray-induced division delay transition point (TPx), at or just prior to prophase, progressed without delay and were selected during the next 50 min (Walters & Petersen, 1968; Schneiderman et al., 1972). The G2- and S-phase cells, located prior to the TPx, sustained a transitory delay and resumed progression into mitosis only after recovery from the radiation insult (Terasima & Tolmach, 1963). However, S-phase cells having incorporated [125I]UdR during the pulse label were prevented from entering mitosis (Schneiderman & Hofer, 1980) and only the X-ray-treated G2 cells resumed progression into mitosis and were selected.