Bone marrow runs the (bone) show.

In this issue of JEM, a paper by Kim et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20211872), asking a simple question through a remarkable alliance of human and mouse genetics, demonstrates that a prevalent hematological condition can lead to osteoporosis. This work is important by virtue of the quality of its results and its implication for the relationship between bone and its marrow.