CDKN1B and pituitary tumor: p27kip1, a cyclin-dependent kinase inhibitor, is down-regulated in corticotroph adenoma [28]; p27 knockdown mice developed pituitary tumors that immunostain for ACTH and p27 germline mutations are associated with human corticotroph adenomas [29, 30], which indicated a key role for p27 depletion in corticotroph tumorigenesis.