The HIV-1 Tat protein activates platelets (90), and mice injected with Tat show a marked increase in plasma PF4 within 1 h (91, 92), suggesting that Tat may be an important trigger of platelet activation to release TGF-β1 prior to the activation of cellular innate responses as viremia begins during acute HIV-1 infection. The gene discussed is PF4; the disease is HIV-1 infection.