The ubiquitin conjugating (ubc) E2 enzyme ubc-9 conjugates the ubiquitin-like peptide sentrin/SUMO-1/PIC1 to target proteins which include the Fas antigen. We show that the mouse genome contains four copies of the ubc-9 gene. These include a structural ubc-9 gene consisting of seven exons which encode a protein identical to human ubc-9, and three intronless processed pseudogenes. The open reading frames (ORF) of two of the pseudogenes, ubc9-psi1 and ubc9-psi2, correspond to the cDNA of ubc-9 and encode for proteins which differ from ubc9 by three and one amino acid substitutions respectively. The third pseudogene, ubc9-psi3, contains many mutations and stop codons. ubc9-psi1 and ubc9-psi2 are flanked by 5'- and 3'-untranslated (UT) regions homologous to those of the structural ubc-9 gene. Both genes contain a polyA tail and direct repeats at both ends suggesting that they arose by mRNA retroposition. Both ubc9-psi1 and ubc9-psi2 are transcribed into mRNA in murine cells. In contrast to ubc9, the protein products of ubc9-psil and ubc9-psi2 fail to bind Fas and to complement an yeast conditional ubc9 mutant. These results suggest that ubc9-psi1 and ubc9-psi2 encode for proteins that may interact with targets that differ from those recognized by ubc-9.