Analysis of μ chain–associated phosphoprotein after treatment with anti-μ. Normal B cells (BC7) and CLL cells were treated with anti-μ (+) or isotype control (−) for 2 minutes. Immunoprecipitations were performed with antiphosphotyrosine antibody 4G10, and μ chain was analyzed by immunoblotting. Immunoblots show results from 1 healthy donor and 6 CLL samples. The immature form (◁) and the mature form (◀) of μ chain are indicated.