CD74, also known as the MHC class II associated invariant chain (Ii), is a type II transmembrane protein which binds to the peptide binding groove of newly synthesized MHC class II alpha/beta heterodimers and prevents their premature association with endogenous polypeptides. CD74 is produced in molar excess over MHC class II and some of this is expressed by an unknown pathway on the cell surface independent of, or in association with, MHC class II molecules. The half life of CD74 on the cell