FLT3LG and osteoporosis: Since osteoporosis is a clinical feature in many advanced FOP patients13,49, bone mass and architecture in the lumbar vertebrae (L4) of AAV-treated Acvr1(R206H)Fl;PDGFRα-cre mice were assessed by microCT and histology (Fig. 5d and f, Supplementary Fig. 10b).