CD3-induced phosphorylation of Syk is augmented in the presence of ZAP-70. The relative phosphorylation levels of ZAP-70 and Syk in Jurkat cells engineered to express either ZAP-70 or Syk alone (ZAP^E/Syk^– and ZAP^–/Syk^E cells, respectively) or together (ZAP⁺/Syk^E) were monitored following their joint immunoprecipitation. Immunoprecipitates were immunoblotted with an αPY mAb, and blots were then reprobed with αZAP-70 and αSyk Abs.