UBE2D4 and Huntington disease: In the HD-associated region, eight UBE2 genes that are part of the ubiquitin conjugating enzymes gene family, UBE2G2, UBE2I, UBE2C, CDC34, UBE2W, UBE2Z, UBE2E3, and UBE2D4, co-expressed with polyQ genes.