In vitro interaction of WASP and SH3-containing proteins. (A) Proteins containing SH2 and SH3 domains were used to affinity-precipitate WASP. Of 5 SH3-GST fusion proteins, 4 (Fyn, Lck, PLC-γ, and Grb2) were able to bind WASP. None of the SH2-GST fusion proteins were able to bind WASP. (B) Lysates of B-LCLs established from 12 WAS patients, some expressing WASP, were incubated in vitro by Fyn SH3-GST fusion protein agarose beads. Only patients with demonstrable WASP showed a band (JS, JH, JG, JD, TS, and possibly CH).