To elaborate the role of macrophages in initiation of HO, we first analyzed HO formation in colony-stimulating factor-1 (CSF-1)-deficient (Csf1−/−) mice, which have macrophage deficiency, as CSF-1 is essential for the survival of monocyte-macrophage–lineage cells. The gene discussed is CSF1; the disease is hyperinsulinemic hypoglycemia, familial, 4.