Because in practice X4 actually infects both naïve and memory CD4+ T cells, in our final model, Model 3, we extend the two-compartment setup of Model 2 to allow X4's infection of memory CD4+ T cells:(Model 3)In this model, kN4 and kM4 are the infection rate coefficients of X4 on naïve (N) and memory (M) CD4+ T cells, respectively, and kM5 is the infection rate coefficient of R5 on memory CD4+ T cells. Here, CD4 is linked to infection.