Increase in the intracellular perforin content seems to be the normal consequence of the process of post-thymic development, and it is also valid in the case of CD4+ T-cell differentiation, since cytotoxic CD4+ T-cells, whose proportions are increased during HIV-1 infection (Appay et al. 2002c), are CD57+ (Figure 5G). Here, PRF1 is linked to HIV-1 infection.