SPAG6 and neoplasm: The tumor suppressor p27Kip1 induces G1-S phase arrest by inhibiting the cyclin E-CDK2 and cyclin A-CDK2 complexes, which are abnormally downregulated in multiple tumors.29, 71 The spag6 knockdown in SKM-1 cells exhibited G1 phase arrest, p27Kip1 upregulation, and cyclin E1 and CDK2 downregulation (Table 1).68