PCI 34051 is a potent histone deacetylase 8 (HDAC8) inhibitor (IC₅₀ = 0.01 μM) with >200-fold selectivity over HDAC isoforms 1, 2, 3, 6, and 10 (IC₅₀s = 4, >50, >50, 2.9, and 13 μM, respectively).