Additionally, bi‐allelic truncating mutations in secreted frizzled‐related protein 4 (sFRP‐4) (Figure 1), which encodes a soluble Wnt inhibitor, have been reported in patients with Pyle's disease, a disorder characterized by cortical bone thinning, limb deformity and fracture 54. The gene discussed is SFRP4; the disease is Pyle disease.