MHC II alpha beta dimers associate with a third polypeptide, the invariant chain (Ii), required for class II molecules to reach the endocytic pathway (Roche et al. 1991). The interaction of Ii with the MHC II alpha beta dimer serves multiple functions. It plays a role in assembly, folding, egress from the ER and transport through the Golgi. Ii exists as a trimer; residues 163-183 of the Ii lumenal domain are involved in covalent cross-linking. Residues 96-104 are critical for association with class II alpha beta dimers (Bijlmakers et al. 1994, Freisewinkel et al. 1993). Residues 89-104 known as CLIP (Class II-associated invariant chain peptide) are the part of the Ii chain that binds antigen binding MHC class II groove, remaining bound until the MHC receptor is completely assembled. This CLIP domain prevents the premature binding of self-peptide fragments present in ER prior to MHC II localization within the endosomal compartment. The ER-resident chaperone protein calnexin rapidly associates with newly synthesized alpha, beta and invariant chains, and remains associated until the final nonamer assembly. The stoichiometry of calnexin in this interaction and the dynamics of association-dissociation are not known. Calnexin may stabilize the free class II chains and regulate their intracellular transport by facilitating the production of transport competent molecules out of the ER (Anderson & Cresswell 1994, Schreiber et al. 1995).