Human antigen presenting cells express three human leukocyte antigen (HLA) class II isotypes (DR, DP, and DQ), which are composed of polymorphic alpha and beta subunits. The combination of polymorphic alpha- and beta-chains results in cis (encoded on the same chromosome) or trans (encoded on different chromosomes) combinations. Since some of the alpha-beta combinations may yield mismatched non-functional alpha-beta heterodimers, it is not entirely clear which type of HLA class II peptide receptors are found on the cell surface of antigen presenting cells. We have developed a combination of biochemical techniques for inspection of the assembly and intracellular transport of isotype matched and mismatched class II heterodimers.