ACVR1 and nephrolithiasis: According to clinical presentation, patients with the classic FOP phenotype, who represent about 92% of cases and all have the canonical ACVR1/ALK2 gene c.617G>A (p.R206H) mutation, show both the typical congenital malformations of the great toe and progressive HO with other common but variable features of FOP such as tibial osteochondromas, conductive hearing impairment, age-depended increased risk of hypercalciuria-related nephrolithiasis, sparse hair and eyebrows, and neuroimaging abnormalities especially involving the pontine region [6].