CCNK and lung cancer: However, Cyclin K knockdown had minimal effects on cell growth and proliferation in Cyclin D1-depleted cells (Figure 7B-C), indicating that Cyclin D1 is the major downstream effector of Cyclin K. Furthermore, we observed that Cyclin D1 depletion accelerated DNA damage, impaired Rad51 foci formation and enhanced radiosensitivity in lung cancer cells (Figure 7D-F), consistent with previous studies showing that Cyclin D1 is a critical regulator of DNA damage repair.