CDKN1B and medullary thyroid gland carcinoma: However, for nine SNPs in the cell cycle regulation (CDKN1A, CDKN1B, CDKN2A, CDKN2B, CDKN2C) and detoxification pathway (CYP1A1m1, CYP1A2*F, NAT2, GSTP1), only single small cohort studies had examined their risk association with MTC.23, 25, 26, 27 Hence the meta‐analysis was performed only for the four RET gene SNPs (G691S, L769L, S836S, S904S) one or more of which are reported in 19 case‐control studies.