Within acidic endocytic compartments Ii is proteolytically cleaved, ultimately freeing the class II peptide-binding groove for loading of antigenic peptides. Ii is degraded in a stepwise manner by a combination of aspartyl and cysteine proteases, following a well defined path with intermediates lip22, lip10 and finally CLIP. The initial Ii cleavage has been ascribed to leupeptin-insensitive cysteine or aspartic proteases, which include aspartyl protease and asparagine endopeptidase (AEP) (Maric et al. 1994, Manoury et al. 2003, Costantino et al. 2008). These proteases generate 22 kDa fragments of Ii (lip22). The trimerization domain of human Ii (residues 134-208) has three possible AEP cleavage sites, Asn148, 165 and 171. Asn171, located at the C-terminal end of helix B, is the demonstrated cleavage site for AEP (Manoury et al. 2003, Jasanoff et al. 1998). This cleavage eliminates the C-terminal trimerization domain of Ii, which causes disassociation of the (MHC II:Ii)3 nonamer and exposes new cleavage sites in the MHC II:lip22 trimers (Villadangos et al. 1999, Guillaume et al. 2008). The residue numbering of Ii given above is based on Uniprot isoform 1.