Defective proximal TCR signaling in ADA-deficient T cells. Intracytoplasmic analysis of phosphorylated ZAP-70 (Y319) in response to cross-linked (1 μg/mL), anti-CD3 plus (10 μg/mL), anti–CD28-mediated activation of CD4⁺ T cells. Dot plots from 2 ADA-SCID patients, before and after GT, and 2 NDs are reported. Results corresponding to frequency of positive cells after 40 seconds of stimulation, in one of 3 independent determinations, from 5 patients and 4 NDs are indicated in the graph. Values corresponding to basal phosphorylation were subtracted (*P < .05; **P < .01).