CDKN1B and neoplasm: Conversely, in H-Ras-driven tumorigenesis, such as the MMTV-H-RasV12-induced breast and salivary gland cancers [16] or the DMBA/TPA skin carcinogenesis model [17], p27 acts as a classical tumor suppressor gene and loss of one p27 allele does not result in enhanced tumor growth.