Previous reports also indicate that thymocytes are more susceptible to in vitro X4 infection compared to infection with R5-tropic HIV strains (Nunes-Cabaço et al., 2015), which correlates with higher expression of HIV co-receptor CXCR4 compared to CCR5 in human thymocytes, while this was not significant in our co-culture model of HIV infection. Here, CXCR4 is linked to infection.