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P ulmonary alveolar proteinosis (PAP) is a heterogeneous disorder of genetic or acquired etiologies. In some cases congenital PAP is associated with hereditary surfactant protein (SP)-B deficiency. To date, the molecular defect in the majority of patients with PAP has not been identified. In mice, PAP has been generated by targeted deletion of the genes for either the GM-CSF/IL-3/IL-5 receptor common beta chain (beta c) or GM-CSF. Here, we describe an expression defect of beta c in three of seven pediatric patients with PAP and in one patient with severe lung disease suspected to be PAP. The patients failed to express normal levels of beta c as shown by flow cytometry. Strikingly reduced or absent function of beta c was demonstrated by ligand binding studies and progenitor clonogenic assays. Analysis of beta c DNA revealed a point mutation from proline to threonine at codon 602 in one patient. Our findings provide evidence that a defect in the expression of a hematopoietic cytokine receptor is associated with human PAP.