Histone deacetylase inhibitors (HDACIs) are promising anti-tumor agents that selectively Induce cell cycle arrest differentiation and/or apoptosis of tumor cells Fundamentally HDACIs are proposed to function by activating the transcription of genes including the potent cyclin dependent kinase inhibitor p21(WAF1) However HDACIs primarily increase p21(WAF1) expression at the post-transcriptional level in HepG2 cells implying that these anti-tumor agents regulate genes at multiple levels Here two novel as-acting elements in the 3' untranslated region (UTR) of p21(WAF1) are identified that control the ability of HDACIs to induce p21(WAF1) mRNA stabilization Collectively these studies highlight the complexity of HDACIs in gene regulation (C) 2010 Elsevier Inc All rights reserved