Ubiquitin (Ub) is among the most phylogenetically conserved proteins known. The primary function of this small, 76 amino acid protein is to clear abnormal, foreign and improperly folded proteins by targeting them for degradation by the 26S proteosome. Many ubiquitin-like proteins function as post-translational protein modifiers, such as members of the SUMO protein family, however some ubiquitin-like proteins regulate protein-protein interactions and cell cycle events, thereby functioning outside of the traditional ubiquitination pathway. UBTD1 (Ubiquitin domain-containing protein 1) is a 227 amino acid protein containing one C-terminal Ubiquitin-like (UBQ) domain. With only one UBQ domain, it is likely that UBTD1 is capable of conjugation to other proteins and mostly functions in similar ways to ubiquitin in the modification of post-translational proteins.