Histone acetyltransferase and histone deacetylase (HDAC) activity regulates the reversible acetylation of lysine residues within histone tails, which results in either transcriptional activation or repression of nearby genes. Compounds that modulate this activity are of interest in cancer chemotherapeutics as well as for treating psychiatric disorders, malaria, and other diseases. Apicidin is a fungal toxin that has broad spectrum activity against Apicomplexan parasites through inhibiting HDACs (IC50 = 0.7 nM). An in vitro activity assay demonstrates apicidin inhibition of HDAC3/NCoR, a class I HDAC, at a much higher potency than for class II HDAC6 (IC50s = 15.8 and 665.1 nM, respectively). Apicidin exhibits antiproliferative activity against various cancer cell lines (IC50s = 0.13-2.36 µM), and at 0.5 - 2 µM it induces selective changes in p21WAF1/Cip1 and gelsolin gene expression, which control cell cycle and cell morphology, respectively.