Lack of IL-2–induced STAT5 phosphorylation by SCID patient EBV-transformed B-cell lines. Depicted are 1-color fluorescence histograms of healthy volunteer or SCID patient cells that had been incubated for 20 minutes with or without 1000 U/mL IL-2, fixed and permeabilized, subjected to indirect staining with Ab specific for tyrosine-phosphorylated STAT5 (phosphoSTAT5) or total STAT5 (STAT5), and analyzed by flow cytometry. Dashed lines represent staining with isotype control Ab and FITC-labeled secondary Ab, whereas solid lines signify staining with the indicated anti-STAT Ab and FITC-conjugated secondary reagent.