Since HIV and KSHV infect different cell types (HIV infects primarily CD4+ T cells and tissue macrophages, and KSHV infects primarily endothelial and B cells), two crucial paracrine mechanisms have been proposed by which HIV promotes KS pathogenesis: (i) promotion of cytokine production from HIV-infected CD4+ T cells and (ii) secretion of HIV proteins, particularly HIV Tat (reviewed in reference 14). Here, TAT is linked to Kaposi's sarcoma.