The R263K substitution was reported at VF in a subtype C-infected participant in the SAILING clinical trial . Thus, in vitro characteristics of a R263K site-directed mutant in the subtype C context was assessed showing a significant decrease in strand-transfer activity for the R263K integrase protein to a greater extent than observed in subtype B . The presence of R263K decreased HIV-1 subtype C infectiousness by 70% compared to wild-type, whereas R263K in subtype B only resulted in a 40% decrease . Thus, the R263K substitution appears to be more deleterious in subtype C than in subtype B.