Progression of the mammalian cell cycle is primarily regulated by phosphorylation/dephosphorylation and synthesis/degradation of many key proteins. Ubiquitin, a soluble protein of 76 amino acids, is enzymatically attached to an e-NH2-Lys in a target protein. Ubiquitination is a hallmark for rapid protein degradation of the target protein in the proteosome (a cytoplasmic complex of proteases). Human homologs of the yeast ubiquitin-conjugating enzymes (Ubc) have been reported, including Ubc9. Ubc9 is 158 amino acids with an apparent molecular weight of 18kDa. Although ubiquitously expressed, the highest levels of Ubc9 are found in testis and thymus. Ubc9 was localized to the synaptonemal complex in male mouse sex chromosomes. Furthermore, Ubc9 interacts with the recombination protein Rad51, thus suggesting an important role for Ubc9 during meiosis.