In many organs, transcription factor networks drive cell fate specification, morphogenesis and growth during organogenesis. Now, on p. 3113, Brendolan et al. report that the same is true for the spleen and provide evidence that Pbx1 is a key co-regulator of spleen ontogeny. By analysing asplenic mice mutant for the transcription factors Pbx1, Hox11, Nkx3.2 and Pod1, the researchers discover that Pbx1 is required for splenic cell fate specification during early embryogenesis and for later progenitor cell proliferation. They show that a crucial function of Pbx1 in spleen development occurs via its genetic and transcriptional interaction with Hox11. And, although Nkx3.2 and Pod1 control spleen development through separate pathways, Pbx1 genetically regulates components of both pathways. Given that loss of Pbx1 causes multiple organogenesis defects, Pbx1 may be a central hierarchical co-regulator of the development of several organs.