Targeted gene disruption of two distinct lineage-restricted hematopoietic transcription factors has provided useful insights into the transcriptional control of platelet production. Absence of either the basic leucine-zipper protein NF-E2 or of the zinc-finger protein GATA-1 in vivo results in severe thrombocytopenia secondary to distinct patterns of arrested megakaryocyte cytoplasmic maturation; in addition, megakaryocyte-selective loss of GATA-1 expression leads to dysregulated proliferation of progenitor cells. The ultrastructure of the defective megakaryocytes suggests that absence of the respective transcription factors impairs biogenesis of platelet-specific granules and proper development and organization of demarcation membranes. In particular, transcriptional targets of NF-E2 may be implicated in the very final stages of megakaryocyte differentiation, which involve the organization and release of platelets. Preliminary characterization of genes that are downregulated in NF-E2-/- megakaryocytes is in progress and is likely to lead to mechanistic insights into thrombocytopoiesis.