CSF2 and pulmonary alveolar proteinosis: Prior studies reporting the relative fractional composition of surfactant phospholipids is normal in pulmonary alveolar proteinosis (PAP) patients5 and Csf2−/− mice6, led to a widely-held belief that surfactant accumulation in PAP is caused by impaired catabolism of phospholipids within alveolar macrophages7, however, to date, no such mechanism has been identified.