GATA-1–deficient platelets lack the characteristic discoid shape and harbor defective marginal bands. (A-B) Representative microscopic fields of resting platelet populations examined by differential interference contrast microscopy, showing the characteristic elliptical shape of normal (A) and spherocytosis of GATA-1–null (B) platelets. (C-D) Indirect immunofluorescence for α-tubulin. Most wild-type (+/+) platelets (C) contain prominent MT rings of relatively uniform diameter, whereas GATA-1 null platelets (D) contain marginal bands that stain faintly and are usually deformed or uncoiled. (E-F) Rapid-freeze electron microscopy of fixed (E) or permeabilized (F) GATA1-null platelets. Surface replicas (E) highlight the heterogeneity in size of GATA1-null platelets. MTs are frequently kinked (arrowhead in F), broken, or separated from the cell periphery.