Chimerism and gain of function of the N642H mutation. (A) The allelic frequencies of the N642H mutation in the different cell types tested. The individual fluorescence-activated cell sorter–sorted cell population was sequenced with digital droplet polymerase chain reaction, and allelic frequencies were calculated as described in Milner et al.¹⁰  (B) Elevated basal and activated phosphorylated STAT5 (pSTAT5) expression in patients’ CD4⁺ T cells. pSTAT5 basal and stimulated by indicated cytokines in patients (solid) compared with different healthy controls (dashed) (top). (C) Normal basal and activated pSTAT5 in CD11c⁺ dendritic cells and CD19⁺ B cells. Patients (solid red) and healthy control (dashed blue) PBMCs were stimulated by interferon α (IFN-α) and granulocyte-macrophage colony-stimulating factor (GM-CSF), and pSTAT5 activation was examined in CD19⁺ and CD11c⁺ cells, respectively. (D) Normal pSTAT3 activation in CD4⁺ T cells stimulated with indicated cytokines in patients (solid) compared with healthy control (dashed). Patient A (top); patient B (bottom).