Two-color flow cytometric analysis of PBMC from case no. 13. Cells were first incubated with FITC-Leu3 (CD4) and biotin-anti–Tac (left), FITC-Leu3 and biotin-315 (OX40) (center), or FITC-anti–Tac and biotin-315 (OX40) (right), washed, and then incubated with PE-conjugated streptavidin. Samples were analyzed using a FACScan.