Abnormal lymphocyte populations detected in STAT3-mutated patients. (A-C) BM biopsy shows abnormally high number of phospho-STAT3–positive lymphocytes both in patient 2 (p.K392R) (A) and, to a lesser extent, in patient 1 (p.K658N) (B). Patient 3 (M394T) was not available for study. In healthy BM, no phospho-STAT3 cells are present (C). (D-F) Flow cytometry results from patient 2 (p. K392R). The majority of lymphocytes were CD3⁺ (A), with 57% of the population expressing TCR-γδ (B). The TCR-γδ⁺ population consisted of CD4⁻CD8⁻ and CD4⁻CD8⁺ T cells. The expression of TCR-γδ was considerably lower in CD4⁻CD8⁻ cells than in CD4⁻CD8⁺ T cells; therefore, 2 populations are seen in the scatter plot. (C). In healthy individuals, TCR-γδ–expressing T cells account less than 6% of all CD3⁺ T cells and the TCR-γδ expression is normally uniform. ×40 magnification, hematoxylin and eosin stain.