Our finding that donor drug use reduces CD63 protein and mRNA, as well as CD9 protein (Figs 4 and 5) and that such SE containing lower levels of CD63 and CD9 were unable to inhibit HIV-1 infection in a pre-treatment SE inhibition model (Fig. 6c) suggest that CD63 and/or CD9 protein in SE may be linked to SE-mediated inhibition of HIV-1 in this model. Here, CD9 is linked to HIV-1 infection.