Histone deacetylases (HDACs) catalyze the removal of the acetyl group from an e-N-acetyl lysine on a histone, resulting in a more tight DNA structure. In this way, they affect chromatin structure and gene expression. They catalyze the same reaction on some non-histone proteins. Inhibition of overexpressed HDACs in tumour cells results in cell cycle arrest, induction of apoptosis, inhibition of angiogenesis and regulation of cellular signaling pathways in a survival-negative manner. Out of 4 known HDAC classes, classes I, II and IV are zinc-dependent enzymes, while class III is NAD+ dependent. All known HDACs inhibitors contain zinc-binding group, responsible for complexation of Zn2+ from the enzyme's active site. Other main structural features include cap group and the linker. With respect to the zinc-binding group, inhibitors can be divided into hydroxamic acids (the most pronounced ability to chelate Zn2+), carboxylic acids, cyclic peptides, and o-aminoanilides. Up to date, five HDACs inhibitors have reached the market: vorinostat, belinostat, and panobinostat (hydroxamic acids), romidepsin (cyclic peptide) and chidamide (o-aminoanilide). They are used in the therapy of T-cell lymphomas and multiple myeloma.