FGFR1 and mesenchymal cell neoplasm: It is caused by ectopic fibroblast growth factor 23 (FGF23) secretion by phosphaturic mesenchymal tumors (PMTs).(1, 2) The primary site of FGF23 action is the proximal renal tubules, where, by signaling through fibroblast growth factor receptor 1 (FGFR1) and the coreceptor KLOTHO, it inhibits renal phosphate reabsorption and 1,25‐dihydroxy‐vitamin D (1,25D) production.