ACVRL1 and bronchopulmonary dysplasia: Indirect evidence for a role of BMP9-ALK1-dependent signaling comes from our previous studies on identifying novel genes in the pathogenesis of BPD using a DNA array experimental setup (Wagenaar et al., 2004) in which we identified the unknown expressed sequence tags with accession numbers AI168935 and AI058357, as being transmembrane protein 100 (TMEM100), a downstream target gene of ALK1 (Somekawa et al., 2012), with a very high differential expression in rat pups with experimental BPD.