Rad 6, also known as Ubiquitin conjugating enzyme E2A, is an E2 enzyme of the three step ubiquitination pathway. Targeting of proteins by ubiquitination occurs in three main steps that involve the activation of ubiquitin by E1 enzymes, the carrying of ubiquitin to the target protein by E2 enzymes, and the ligation of ubiquitin to the target protein by E3 enzymes. Ubiquitination provides an important post-translational means to regulate protein abundance and activity. Rad6 has been demonstrated to be important to DNA damage repair. Mutations in Rad6 cause an X-linked mental retardation syndrome.