GATA-1–deficient megakaryocytes. Phase-contrast micrograph (a), acetylcholinesterase staining (b), and indirect immunofluorescence with a rabbit antimouse platelet antiserum (c) each show the rarity of proplatelets among megakaryocytes cultured from GATA-1–deficient mice. Control megakaryocytes (not shown here; see Figs 1 and 2) displayed abundant proplatelets by each of these techniques. For (a), (b), and (c), original magnifications × 200.