GDF2 and bronchopulmonary dysplasia: If negative effects of treatment with BMP9 in human infants are absent, extrapolation of the beneficial effects of BMP9 treatment in rat pups with experimental neonatal chronic lung disease to preterm infants with respiratory failure may result in a novel therapeutic treatment option for preterm infants with severe BPD by improving aberrant alveolar development and reducing pulmonary inflammation and fibrosis, which are major contributors to mortality and morbidity.