Clinical and laboratory findings in 2 children with a somatic gain-of-function mutation in STAT5b. The urticarial skin rash (A and E), underlying histological features (B, C, F, and G), and absolute eosinophil count (AEC) over time (D and H) are shown for both children. Photomicrographs show a perivascular lymphocytic infiltrate (B; original magnification ×5, hematoxylin and eosin stain) with edema and scattered eosinophils (C; original magnification ×20, hematoxylin and eosin stain) and a deep subcutaneous lymphohistiocytic infiltrate involving the lobules consistent with a lobular panniculitis (F; original magnification ×4, hematoxylin and eosin stain) with no evidence of rimming of the lymphocytes and scattered eosinophils at high power (G; original magnification ×40, hematoxylin and eosin stain). Arrows indicate representative eosinophils. The upper limit of the normal range for AEC is indicated by a dashed line.