T-cell early developmental stages and alternative options: GATA-3 is depicted as an intrathymically induced, specific inhibitor of access to the B-lineage pathway. Recognized stages in early intrathymic T-cell development are shown, along with the degrees of access to the B-cell program and other alternative programs that the cells can demonstrate at these stages if the extrinsic barrier of Notch/Dll4 signaling (red line) is removed. Red bricks represent GATA-3 when expressed at critical levels, and purple bricks represent other T-lineage commitment factors (eg, Bcl11b⁶ ) that are activated later. Not shown are the positive roles of GATA-3 that permit the efficient generation of DN2 cells. Full commitment is complete by the DN2b stage.⁶