The DE genes in KEGG pathway of cytokine-cytokine receptor interaction were selected for qRT-PCR confirmation; this is because this pathway was identified as being enriched in comparison between HIV and HCV mono-infections (R = 9.06, 4 genes: CXCL16; TNFRSF9; CCR4; and CX3CR1), as well as in comparison between HIV/HCV co-infections and the HCV mono-infections (R = 25, 8 genes: CXCL16; TNFRSF9; CCR4; CX3CR1; IL13RA1; IFNGR2; CD40LG; and PPBP). Here, PPBP is linked to infection.