We identified a novel gene PCCX1 that encoded a nuclear protein carrying a PHD finger, a CXXC domain, and an acidic region. The CXXC domain was found to be sufficient for binding to DNA. The acidic region exhibited a high transactivation ability, but the full-length protein was inactive due to regions which inhibited the acidic region, including the C-terminal region. We examined the expression of PCCX1 during cellular aging and immortalization of SV40-transformed human fibroblasts. PCCX1 mRNA was expressed constitutively through stages of cellular aging and immortalization, but at the protein level, a shorter form lacking the C-terminal region appeared as the cells approached crisis. These results suggested that PCCX1 was activated by proteolytic cleavage, which removed the C-terminal inhibitory region.