Numerous transcription factors (TFs), including PU.1 and Scl, are known to play important roles during haematopoiesis, but how these act within wider TF networks is unclear. Now, Berthold Göttgens and colleagues use transcription activator-like effectors (TALEs) to manipulate the expression of PU.1 and Scl and determine how these TFs function during developmental haematopoiesis (p. 4018). They first show that the modulation of PU.1 expression affects cell fate decisions during embryoid body haematopoiesis; PU.1 upregulation, for example, drives haematopoietic commitment but causes a loss of proliferative ability, whereas PU.1 repression inhibits the maturation and differentiation of early haematopoietic cells. They further report, using single-cell gene expression analyses, that TALE-induced PU.1 expression is associated with changes in the expression of several other haematopoietic genes, suggesting that early activation of PU.1 expression drives a haematopoietic programme at the expense of endothelial gene expression. Following on from this, the researchers show that the PU.1-14kb enhancer is active in the mid-gestation dorsal aorta in vivo, and that PU.1 is detectable in the early haemogenic endothelium. Together, these studies uncover a novel role for PU.1 during haematopoietic specification and highlight the use of TALEs in understanding developmental TF networks.