TAT and Kaposi's sarcoma: Low concentrations of extracellular Tat – similar to those measured in HIV-infected humans, i.e. 2–40 ng/ml [12] – have been shown to promote angiogenesis [13], to stimulate growth and motility of cultured Kaposi sarcoma cells [14], to induce monocyte chemotaxis through the synthesis of platelet-activating factor [15], to stimulate vascular permeability and the recruitment of mononuclear cells [16].