CSF2 and pulmonary alveolar proteinosis: Homozygous loss-of-function mutations in CSF2RB have been described in pulmonary alveolar proteinosis, in which there is completely absent GM-CSF–induced STAT5 phosphorylation.47, 48 None of the patients in our study with the CSF2RB frameshift showed a pulmonary phenotype, presumably because of the existence of some residual CSF2RB activity.