PECAM1 and neutropenia: Prospective isolation of highly purified mesenchymal (niche-CD31−CD51+Sca1+) and endothelial (niche-CD31+Sca1+) cells (Supplementary Fig. 6A) showed an increased frequency of these endosteal niche cells in neutropenia (Supplementary Fig. 6B, C), consistent with a previous report of niche expansion upon short-term neutrophil depletion6.