Zap‐70, a crucial molecule for the selective activation of T cells, through its interaction with the ζ chain of the TCR/CD3 complex, is a tyrosine kinase. This well studied molecule has gained a renewed interest upon the demonstration of its transduction and expression in B cells from patients with chronic lymphocytic leukemia. Here the major characteristics of this cytosolic protein are reviewed, as well as its expression in various cell types and some indications about its detection. © 2006 International Society for Analytical Cytology