ACVR1 and fibrodysplasia ossificans progressiva: Lastly, it has been reported that endothelial cells expressing mutant activin 1 receptors (ACVR1) can trans differentiate into osteoblasts, an event that may be critical in the pathogenesis of a devastating disease such as Fibrodysplasia Ossificans Progressiva (FOP) (30, –32).