“…Additionally, induction may result from exogenous signals, such as inducers of differentiation, leading to the inhibition of proliferation (Sherr and Roberts, 1995;Harper and Elledge, 1996). p21 (known as WAF1, SDI1, MDA-6, CIP1 and CAP20) was the ®rst CDKI isolated, based on interactions with CDK2 (Gu et al, 1993;Harper et al, 1993) and PCNA (Xiong et al, 1992), as well as induction associated with p53 tumor-suppressor expression (El-Deiry et al, 1993), terminal di erentiation of human melanoma cells (Jiang and Fisher, 1993), and senescence of human diploid ®broblasts (Noda et al, 1994). p21 has not only been shown to inhibit the activity of a wide array of CDK-cyclin complexes (Xiong et al, 1993;Harper et al, 1995), but also inhibits DNA replication through its interaction with PCNA (Waga et al, 1994;Flores-Rozas et al, 1994).…”