Defective reorganization of β1-tubulin^—/—and GATA1-deficient marginal bands with thrombin-induced activation. Anti–α-tubulin immunostaining of wild-type (A-C), β1-tubulin^—/— (D-F), and GATA1-null (G-I) platelets stimulated with thrombin and examined under low (A,D,G) or high (B-C,E-F,H-I) magnification. Wild-type platelets reorganize the MT coil into tight central rings with scant radiating fibers, whereas β1-tubulin– and GATA1-null platelets contain unraveled or completely disorganized MTs arranged randomly or in short bundles.