['There is evidence that gag contributes to protease inhibitor susceptibility in treatment-experienced patients. Moreover, protease inhibitor resistance-associated mutations can arise in gag in the absence of protease mutations in vitro. We wished to assess the contribution of full-length Gag to protease inhibitor susceptibility in viruses unexposed to protease inhibitors, in particular from the most common HIV-1 subtypes, namely subtype A and C.', 'We compared the drug resistance profiles of subtype A and C cognate gag-protease (from viruses not previously exposed to protease inhibitor) to protease combined with a generic subtype B gag as in routine phenotypic testing.', 'We amplified gag-protease sequences from plasma-derived virus or molecular clones, and used a single cycle transfection-based drug resistance assay to compare the fold changes in the concentration of drug required to inhibit 50% of viral replication of these viruses to a generic subtype B. We made a series of chimeras to explore phenotypes further.', 'In some cases, use of protease sequences without the cognate gag overestimated susceptibility to protease inhibitors, in particular to lopinavir. We provide evidence that gag sequences from wild-type viruses can contribute as much as 14-fold reduction in susceptibility to lopinavir, and that cognate protease can balance this by partially restoring susceptibility.'] Does full-length HIV-1 Gag determine protease inhibitor susceptibility within in vitro assays?