CDKN2C and neoplasm: Further, using primary cultures of pineal tumor cells, we found that both gamma irradiation and treatment with etoposide resulted in increased p53 phosphorylation and in p53-dependent increases in p21Cip1 and 14-3-3 in Irbp-Cyclin D1, p18Ink4c -/- but not Irbp-Cyclin D1, p53 -/- tumor cells [Figure 5E].