CDKN1A and neoplasm: Our data suggests that the limited growth potential of primary tumor cultures results from two major proliferative barriers, (a) telomerase inactivation, potentially leading to telomere attrition, and (b) negative growth signaling by upregulated TGFβ resulting in a significant increase in transcription of the CDKI, p21CIP1/WAF1, consistent with the findings of induction or stabilization of this gene in a variety of immortalized cells exposed to exogenous TGFβ [16,17].