TPO treatment increased GATA-1 content in Mks from GATA-1^low mice while decreasing GATA-1 content in Mks from wild-type littermates. GATA-1 immunostaining of spleen sections from untreated (A-B) and TPO-treated (day 7 after 100μg TPO/kg per day for 5 days) (C-D) wild-type and GATA-1^low mice, as indicated. Mks in spleen sections from untreated wild-type and TPO-treated GATA-1^low mice strongly reacted with the GATA-1 antibody, while those from untreated GATA-1^low and TPO-treated wild-type animals did not. Similar results were observed in sections from marrow and liver from these same animals (not shown). Original magnification × 400. Numerical aperture of the objective lenses: 1.30.