CDKN1A and giant cell tumor: However, p21Cip1 was also found to be highly expressed in PCs, although 80% of the analyzed tumors were hormone-secreting.[37] Similar to giant cell tumors, upregulation of p21Cip1 appears to promote cyclin D1 accumulation, contributing to tumor formation.[38] Thus, the results regarding the role of p21Cip1 in pituitary tumors are conflicting, and it is unclear whether p21Cip1 is involved exclusively in hormonal secretion, or also in other mechanisms underlying PC initiation and progression.