Ectopic ossification in ttw mice is reportedly caused by abnormally elevated vitamin D signaling, and phenotypes seen in ttw mice are completely abrogated when ttw mice are crossed with vitamin D receptor (VDR)-deficient mice [14].Ectopic ossification is also observed in patients with fibrodysplasia ossificans progressive (FOP) [24,25]. The gene discussed is VDR; the disease is fibrodysplasia ossificans progressiva.