Despite observing similar HLA-DR and Gag retention/reduced virus release effects as Finzi et al.(2006) , our results differed in that we did not observe the requirement for intact HLA-DR a and b-chain cytoplasmic tails for the induction of Gag retention. In our hands, when the ubiquitinatible lysine residues in the HLA-DR b-chain were mutated or the cytoplasmic tails of the HLA-DR dimer were completely removed, there was no alleviation of Gag retention or virus release. Beyond the obvious differences between the previous work [8] and our own (i.e. provirus construct and cell type), our mutant data may differ from theirs because arginine residues were substituted for lysine 215 and tyrosine 220 residues in the HLA-DR a and b chain, respectively, in order to ensure stabilization of the truncated HLA-DR molecule at cellular membranes. These differences in mutant constructs may potentially explain discrepancies in our results.