ABCG1 and pulmonary alveolar proteinosis: In keeping with this, ABCG1-deficient macrophages can also be found in patients with pulmonary alveolar proteinosis (PAP) – a rare disease characterized by the build-up of surfactant in the alveolar space (Borie et al., 2011) – in conjunction with autoantibodies against GM-CSF blocking its signaling cascade (Thomassen et al., 2007).