Plasma membrane-associated nonameric complexes (MHC II alpha/beta/Ii complex) are rapidly internalized and delivered to late endosomes (LEs) and lysosomes. The dileucine-based signal present in the cytoplasmic tail of Ii is required for sorting of the nonameric MHC II-Ii complex from the plasma membrane to peptide loading compartments. These signals promote rapid internalization by recognising and binding to clathrin adaptor AP-2, a scaffolding-protein complex that brings together components of the vesicle-formation machinery. AP-2 is an essential component of an endocytic clathrin coat and participates in initiation of coat assembly. The critical role of AP2 in delivering MHC II:Ii complex to antigen processing compartments came from RNA interference studies targeting clathrin and AP2. The knockout of AP2 profoundly inhibited MHC II:Ii complex internalization and resulted in the accumulation of Ii at the surface (Dugast et al. 2005, McCormick et al. 2005).