STAT5 phosphorylation is diminished in all examined cases. STAT5 phosphorylation was determined by FACS analysis in monocytes (n = 6) and granulocytes (n = 5) after stimulation with GM-CSF. One of the patients’ parents was used as a control in each experiment. A. In monocytes, almost no MFI increase is visible in PAP patients as compared to controls. The right panels demonstrate an exemplary MFI shift in a control, whereas the patient’s signal remains unchanged after stimulation. B. A similar pattern is seen in granulocytes.