SP1 and neoplasm: HDACs act in tumor cells mainly through the following actions: through chromatin remodeling as in acute promyelocytic HDACs act on cycle-related factors, such as the p21 cytokine-dependent kinase inhibitor CDKI class, an important one [18], and class I HDACs are dependent on Sp1/Sp3 to inhibit p21 expression in colon cancer [19, 20].