Selected phosphorylated substrates are required for apoptosis in WEHI-231. Induction of tyrosine phosphorylation in WEHI-231 expressing Igα-, Igβ-, or Igα and Igβ-containing chimeric receptors (top). Wt and transfected cells were stimulated through chimeric proteins (Ch), endogenous IgM (Ig), or both (Ch/Ig). Phosphorylation of Syk (arrow) is preferentially induced in cells undergoing apoptosis (bottom).