SP1 and cancer: We found that the suppression of osteoblast proliferation by Hdac4 ablation was attributable to its negative effects on the expression of cell cycle-related genes, including Cdkn1a, Ccna2, Ccnb1, Cdk1, and Pcna. Hdac4 knockdown led to the upregulation of negative regulators of the cell cycle, such as Cdkn1a. Studies have shown that HDAC4 represses CDKN1A expression in human cancer cells in a SP1-dependent manner (Qian et al., 2006).