Annexin V staining confirmed that WAS lymphocytes underwent apoptosis at a greater frequency than normal lymphocytes. Representative data indicate that a higher percentage of WAS lymphocytes had externalized phosphatidylserine and bound annexin V compared with normal lymphocytes. Furthermore, a higher percentage of lymphocytes from WAS patients were inviable, no longer excluding PI, relative to lymphocytes from normal individuals. Apoptotic cells are represented by events in the lower right-hand quadrant, with viable and dead cells depicted by events in the lower left-hand and upper right-hand quadrants, respectively. The fraction of cells undergoing apoptosis and the fraction of dead cells are indicated at the right of each plot. (B) Both methods of analysis, annexin V staining and TUNEL, indicated that WAS lymphocytes underwent apoptosis at a greater frequency than normal lymphocytes. Samples from 4 different WAS patients (no. 1 through 4) and 6 different normal controls were analyzed. Statistical analysis by an unbalanced repeated measures ANOVA showed that the differences between WAS and normal levels of apoptosis as measured using annexin V were significant after in vitro incubations of 48 and 96 hours, as indicated by the asterisks (P = .0013 and .0181, respectively). TUNEL data (shown in Fig 1) is duplicated for easy comparison.