Human Ubc9 is homologous to ubiquitin-conjugating enzymes（E2s). However， instead of conjugating ubiquitin， it conjugates a ubiquitin homologue， small ubiquitin-like modifier 1（SUMO-1). And hUbc9 retains striking structural and functional conservation with yeast Ubc9. The ubiquitin-dependent protein degradation system has been recognized as a complete enzymatic pathway that is responsible for the selective degradation of abnormal and short-lived proteins. The conjugation of ubiquitin requires the activities of ubiquitin-activating（E1) and –conjugating（E2) enzymes.
