The p63 gene, a member of the p53 family, is expressed as TA and ⌬N isoforms. ⌬Np63 is the predominant isoform expressed in cells of epithelial origin and frequently overexpressed in cancers. However, what regulates p63 expression is uncertain. Here, we showed that ⌬Np63 is regulated by the transcription factor DEC1, a p53 family target. We also showed that the ability of DEC1 to regulate ⌬Np63 is enhanced by histone deacetylase (HDAC) inhibitors or knockdown of histone deacetylase 2 (HDAC2). Consistent with this, we found that DEC1 and HDAC2 physically interact and knockdown of HDAC2 leads to increased binding of DEC1 to the ⌬Np63 promoter. Interestingly, we found that growth suppression induced by HDAC inhibitors is attenuated by ectopic expression of DEC1 in a ⌬Np63-dependent manner. In addition, we showed that ectopic expression of DEC1 inhibits, whereas knockdown of DEC1 promotes, keratinocyte differentiation via modulating ⌬Np63 expression. Finally, we showed that DEC1 cooperates with HDAC inhibitors to further decrease keratinocyte differentiation. Together, we conclude that ⌬Np63 is a novel target of DEC1 and HDAC2 and modulates the efficacy of HDAC inhibitors in growth suppression and keratinocyte differentiation.p63 shares a similarity with p53 and p73 in sequence-specific DNA binding, activation, and tetramerization domains (1). Like p53, p63 is a transcription factor and capable of inducing a diverse array of target genes responsible for p63 function in inducing cell cycle arrest, differentiation, and apoptosis, and in maintaining the proliferative potential of epithelial stem cells (2-7). Due to the presence of the upstream P1 promoter and the P2 promoter in intron 3, the p63 gene is expressed as TA and ⌬N isoforms (1). TAp63 contains the N-terminal transactivation domain (TA) conserved among the p53 family (1). ⌬Np63 contains 13 unique residues, which together with the prolinerich domain constitutes a novel activation domain (3,8). In addition, RNA splicing leads to production of at least five alternatively spliced isoforms, ␣, , ␥, ⑀, and ␦ (9).It has been shown that mice deficient in p63 are defective in epidermal development, including severe limb malformation and an almost complete absence of mammary glands, skin, teeth, and hair (7, 10, 11). Another study (12) showed that mice heterozygous for p63 are prone to develop an increased tumor burden and metastasis rate, which is compounded in mice harboring heterozygous alleles of p53 and/or p73. Consistent with this, evidence showed that p63, especially ⌬Np63, is found to be amplified and/or overexpressed in multiple tumors and correlated with tumor progression and poor prognosis (13). In addition, overexpression of ⌬Np63 promotes cell proliferation in vitro and tumor growth in vivo (14). These results suggest that TAp63 and ⌬Np63 have opposing functions in tumor suppression. However, somatic mutations of p63 in tumors are rarely detected (15). Thus, the control of p63 activity is likely at the level of p63 expression.DEC1, 2 also named Stra13 (st...