The CKI p21Cip1, which mediates cell cycle arrest, apoptosis, and differentiation, was previously shown to be a crucial target of the transcriptional corepressor HDAC1 in mouse ES cells (Lagger et al., 2002) and human tumor cells (Lagger et al., 2003; Gui et al., 2004). This evidence concerns the gene HDAC1 and neoplasm.