Tat is detectable on the surfaces of both gp120-positive and gp120-negative cells in vivo. Two PBMC samples (ARRA and VEBE) and 2 lymph node biopsies (UMB00-003 and GW97-91) were simultaneously stained with NT3 2D1 IgG₁ anti-Tat and CRA3 IgG_2a anti-gp120 antibodies, followed by detection with anti-IgG₁–TRITC and anti-IgG_2a–FITC secondary antibodies. Arrows point at specific staining either for Tat (red), gp120 (green), or colocalization of the 2 proteins (yellow). Control pictures are staining with secondary Ab only.