Dual requirements for GATA-1–dependent megakaryocyte development. We conclude that GATA-1 controls 2 sets of genes: One group, including cyclin D1, regulates growth and polyploidization. A second class of targets, which includes genes such as NF-E2, PF4, and c-mpl, drives terminal maturation and platelet biosynthesis. The observation that ectopic cyclin D1–cdk4 can cause increased size and DNA content without concomitant expression of NF-E2, PF-4, and c-mpl indicates that the 2 processes (ie, polyploidization and terminal maturation) can be uncoupled from one another.