ZAP-70 (zeta associated protein 70) is a tyrosine kinase of the Src family of tyrosine kinases. It was initially identified as a partner of the zeta chain of the CD3 complex associated to the T-cell receptor on T-lymphocytes. ZAP-70 shares homology with spleen-tyrosine kinase (Syk), a kinase involved in B-cell activation. Both ZAP-70 and Syk have been shown to be crucial to B- and T-lymphocyte development. In T-cells ZAP-70 functions to transduce the signaling cascade activated by T-cell receptor engagement and activation.