In tissues, modest increases of caspase 3 expression in CD4 T cells were also observed by day 10 following SIVmac251 infection in colon (P = 0.04) and spleen (P = 0.03) with less than 4% tissue-resident CD4 T cells undergoing apoptosis (Fig. 3B; Fig. S4B). Here, CD4 is linked to infection.