PreB cells are moving on.

In this issue of JEM, Fistonich et al. (https://doi.org/10.1084/jem.20180778) address how the bone marrow microenvironment supports diverse lineages through multiple developmental stages. Differential motility between pro- and preB cells results in differential IL-7 exposure, and, intriguingly, stromal cells respond to abnormal B cells by reducing Il7.