Syk is required for CD45-independent target cell killing. (A-B) ADCC induced by coating a chromium-labeled RMA cell line with anti-CD90 in the absence (♦) or presence of CD16-blocking antibody 2.4G2 (◇); cells alone served as an additional control (□). (A) Comparison between NK cells that are CD45 deficient and that are both CD45- and Zap70-deficient. (B) Comparison between NK cells that are CD45-deficient and that are both CD45- and Syk-deficient from chimeric mice. Data are representative of 3 independent experiments.