UBE4A (ubiquitin conjugation factor E4 A), also known as E4, UFD2 or UBOX2, is a 1,066 amino acid protein that functions in the multiubiquitin pathway of protein degradation. Expressed in cortical neurons and in tubular kidney cells, UBE4A is the human homolog of the Sacch-aromyces cerevisiae UFD2 protein and functions with the UBE (ubiquitin enzymes) proteins to catalyze ubiquitin chain assembly. UBE4A may be in-volved in cell growth and differentiation and can act as an autoantigen in scleroderma, a disease characterized by excessive deposits of collagen in the skin or other organs. Two isoforms of UBE4A exist due to alternative splicing events.