USP is a family of cysteine peptidases, which belongs to the MEROPS peptidase family C19 (ubiquitin-specific protease family, clan CA). Families within the CA clan are loosely termed papain-like as protein fold of the peptidase unit resembles that of papain, the type example for clan CA. Predicted active site residues for members of this family and family C1 occur in the same order in the sequence: N/Q, C, H. The type example is human ubiquitin-specific protease 14 (1).

1. http://www.ebi.ac.uk/interpro/entry/IPR001394