RET and medullary thyroid gland carcinoma: To directly assess the effect of CDK5 activity on the expression of cell cycle regulators in human MTC, we compared cell cycle protein expression in a human sporadic, non-RET mutated MTC cell line, MTC-SK [24], which was transfected with either a construct encoding kinase-dead CDK5 (Kd-CDK5) or with a control plasmid.