Anti–GATA-1 antibody staining of spleens. Spleens isolated from either a heterozygous mutant female mouse with marked splenomegaly (C and D) or a wild-type mouse (A and B). Many cells in the red pulp (R) of the spleen of the wild-type mouse were clearly GATA-1 positive. Note that both proerythroblasts and megakaryocytes were GATA-1 positive. In contrast, accumulating proerythroblasts and megakaryocytes did not stain for GATA-1 in the spleens of heterozygous mutant female mice (C and D). R, red pulp; W, white pulp; Meg, megakaryocytes. Arrows stand for normal GATA-1–positive cells. Original magnifications × 10 (A and C) and × 80 (B and D).