JAK3 FERM mutations identified in ATLL rescue T-cell differentiation in JAK3 knockout hematopoietic progenitors. (A) Schematic shows human JAK3 protein with 7 JAK homology (JH) domains; JH6-7 comprise the FERM (gray). Residue numbers and major structural domains are shown. Sequencing revealed 3 mutations (boxes) in 4 of 36 ATLL patients analyzed. L156P and R172Q were identified in 1 patient each and E183G was found in 2 patients. The mutations occurred in residues highly conserved across mammalian species. (B) Western blot for JAK3 (106 kDa), STAT5A (92 kDa), and phospho-STAT5A (Y694) from T-cell progenitor cell lysates grown on OP9-DL1 at day 14; no lysate was recovered from untransduced cells. (C) Graph shows cell counts over time in OP9-DL1 coculture. JAK3^−/− progenitor cells transduced with empty MSCV failed to proliferate and differentiate. Proliferation was rescued for those progenitors transduced with WT or mutant JAK3. (D) Graph shows the percentage of double-positive (CD4⁺CD8⁺⁾ cells arising in OP9-DL1 coculture as determined by flow cytometry; bars from left to right show counts at 14, 21, and 28 days. The OP9-DL1 data are representative of 2 independent experiments (mean ± SEM).