I SPI1 something needed for B cells.

In this issue, Le Coz et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20201750) describe a novel immunodeficiency syndrome caused by mutations in SPI1. Through a series of in-depth studies, the authors provide insights into how SPI1 affects blood lineage specification, highlighting the important role of master transcription factors as cellular fate determinants.