Ubiquitin-conjugating enzyme E2 A (also known as HHR6A or UBE2A), encoded by human DNA repair genes HHR6A, belongs to the ubiquitin-conjugating enzymes (E2 enzymes) family and is likely to be involved in postreplication repair and induced mutagenesis. UBE2A is described as a CDK2 substrate. It is the human homologue of the product of the Saccharomyces cerevisiae RAD6 / UBC2 gene, a member of the family of ubiquitin-conjugating enzymes. In vivo, HHR6A phosphorylation peaks during the G2/M phase of cell cycle transition, with a concomitant increase in histone H2B ubiquitylation. Mutation of Ser120 to threonine or alanine abolished UBE2A activity, while mutation to aspartate to mimic phosphorylated serine increased UBE2A activity 3-fold. A mutation of UBE2A is consisdered as the cause of a novel X-linked mental retardation (XLMR) syndrome that affects three males in a two-generation family.