ACVRL1 and vascular malformation: Conditional deletion of both Alk1 alleles in adult mice by tamoxifen-inducible Cre resulted in AV fistula formations and spontaneous hemorrhage mostly in the lung, gastrointestinal track, and uterus, but not remarkably in the brain, although de novo vascular malformation lesions developed upon induction of skin wounding in these mice [26].