The lip10 fragments bound in the nonameric complex are processed leaving only the CLIP fragment (81-105) bound to the MHC II peptide binding groove. Three cysteine proteases have been shown to be capable of digesting lip10, each with a different expression pattern among different APC. Cathepsin (Cat) S digests Ii in B cells and dendritic cells, thymic epithelial cells use Cat L and Cat L, while Cat S and F are active in macrophages (Villadangos, 2001; Bryant et al, 2002). Cat S appears to be the major enzyme involved as demonstrated by the use of specific inhibitors and of knockout experiments (Stumptner-Cuvelette et al. 2002). Following lip10 digestion, the MHC-like molecule HLA-DM induces the exchange of CLIP fragment for a highly diverse array of antigens (Denzin & Cresswell. 1995).