Haematopoiesis – the formation of blood cells – is regulated by a number of ubiquitous and tissue-specific transcription factors, but the extent of interplay between these factors is unclear. Sp1 is a transcription factor that is ubiquitously expressed and regulates the expression of thousands of genes, and it has been shown that Sp1-deficient mouse embryos die during early development. Now, on p. 2391, Sjaak Philipsen, Constanze Bonifer and colleagues reveal a crucial role for Sp1 during the early stages of haematopoiesis. Using mouse embryonic stem cells (ESCs) that express a DNA binding-deficient variant of Sp1, the researchers first show that Sp1 activity is required for the differentiation of ESCs to hematopoietic lineages; the cells can progress through most steps of blood cell development but are unable to complete terminal differentiation. Furthermore, they demonstrate that gene expression in Sp1-deficient ESCs becomes progressively deregulated as they differentiate. In particular, they report, some Cdx and Hox family genes that are direct targets of Sp1 are downregulated at an early stage of differentiation, and this is followed by the progressive deregulation of other genes that are implicated in haematopoiesis, suggesting that the effects of Sp1 deficiency are cumulative. Together, these findings identify a crucial role for Sp1 during haematopoiesis and provide detailed insight into the hierarchy of the transcriptional network that drives blood cell formation.