This technology includes the identification and use of novel ACRV1/ALK2 inhibitors for the treatment of fibrodysplasia ossificans progressiva (FOP), an autosomal-dominant rare disease that affects one person in every 1-2 million. FOP is characterized by malformation of the great (big) toes during embryonic development and by progressive heterotopic endochondral ossification (HEO) postnatally, which leads to the formation of a second skeleton of heterotopic bone. Individuals with the classical features of FOP have the identical heterozygous activating mutation (R206H) in the gene encoding ACRV1 (also known as ALK2), a BMP type 1 receptor.