Histone demethylase (HDM) proteins are involved in the post-translational modification called histone demethylation which entails the removal of methyl groups in modified histone proteins. Protein arginine demethylase (PRDM) catalyzes the removal of methyl groups from arginine residues while lysine demethylase (KDM) catalyzes the removal from lysine demethylase (KDM).