GATA1-deficient platelets lack β1-tubulin. Representative photomicrographs of platelet populations examined by Texas red–phalloidin (A,C) staining and indirect β-tubulin immunofluorescence (B,D-F). Wild-type (+/+) resting platelets show normal, bright staining for β1-tubulin (B), whereas GATA1-null platelets contain no detectable signal (D). Immunostaining with a mixture of β2 and β5 antibodies demonstrates that +/+ (E) and GATA1-null (F) resting platelets contain other β-tubulin isoforms. (G) Immunoblot analysis confirms absence of β1-tubulin in GATA1-deficient platelets. β1-tubulin knockout or wild-type cells and reaction with glyceraldehyde 3-phosphate dehydrogenase (GAPDH) antibody serve as controls.