STAP1, also known as signal-transducing adaptor protein 1, appears to functions as a docking protein acting downstream of Tec tyrosine kinase in B cell antigen receptor signaling. The protein is directly phosphorylated by Tec in vitro where it participates in a positive feedback loop, increasing Tec activity. Recombinant human STAP1 protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.