RAD21 and cancer: Taking the previous reports together, several genes are highly expressed in cancer cells and integrally involved in SCLC pathogenesis, including NDC80, BUB1B, KIF2C, CDC20, MAD2L1, TOP2A, PCNA, RFC4, CHEK1, TYMS, MCM2, CDKN3, MCM3, CDC6, KIF11, MSH2, and RAD21 [12].