CLOCK and osteosarcoma: Furthermore, CLOCK knockdown in human U2OS osteosarcoma cells resulted in abnormal cell cycle checkpoint response following irradiation and increased sensitivity to mitomycin C, indicative of a CLOCK-dependent response to repair DSBs (Cotta-Ramusinoet al., 2011).CLOCK was also found to localize to laser-induced DSBs in U2OS cells, suggesting a potential direct role in the cellular signaling machinery required for DSB repair (Cotta-Ramusinoet al., 2011).