Radioresistance varies in different phases of the cell cycle. Cells are most resistant in G0, in early G1, and in the late S phase of the cell cycle. Cells are most radiosensitive in late G1, G2, and throughout the M phase of the cell cycle. Radiation resistance in the S phase is thought to be due to an elevated amount of DNA synthesis and repair enzymes, as well as elevations in the intracellular levels of glutathione (a free radical scavenger). Cell cycle blockage in the G1 phase of the cell cycle after ionizing radiation is believed to allow time for the recognition/repair of DNA damage prior to the initiation of DNA synthesis. Cells are most sensitive in the G2/M phase of the cell cycle, presumably because there is no time for adequate repair before chromosome segregation takes place.