CSF2 and connective tissue disorder: The childhood PAP classification scheme generally includes the following categories (Bush and Pabary, 2020): Disorders related to surfactant protein metabolism (involving SFTPB, STFPC, ABCA3 and NKX2-1 mutations); GM-CSF receptor gene mutations (including CSF2RA/CSF2RB mutations); other genetic disorders (including ARS, STING, COPA and GATA2 mutations); metabolic disease (such as lysinuric protein intolerance, Niemann-Pick disease); secondary PAP (such as immunodeficiency, connective tissue disease), and autoimmune PAP (caused by IgG autoantibodies to GM-CSF) (Table 2).