Analysis of human MTC tissues from sporadic and hereditary cases showed that the cell cycle proteins, CDK2, cyclin-D1, p18INK4c, p19INK4d and p21CIP/WAF1 were consistently overexpressed in human sporadic, but not hereditary, forms of MTC (Table 1). Here, CDKN2D is linked to medullary thyroid gland carcinoma.