The iUUCD contained important proteins related with ubiquitin and ubiquitin-like conjugation systems, including E1s, E2s, E3s, DUBs, UBDs and ULDs. According to the function or conserved activity domain, iUUCD proteins were classified into 74 families. 64 of 74 families shared demonstrated 3D structures. These 3D images were used as logos for their families with two blank pictures used for other 10 families. Besides, all families were showed on the treeview. Here the page shows all families of the database. Users could click the names on the left treeview or the logos on the right figure to browse each family. Users could also Browse by classifications of enzymes or Browse by classifications of ULDs.