Identification of CD45 mutations in T-ALL. (A) DND-41 is sensitive to JAK1 knockdown. SUP-T1 and DND-41 cells were electroporated with a JAK1 targeting or scrambled control siRNA. Cell proliferation was normalized to scrambled control. The average ± SEM of 3 repeats is shown. *Statistical significance. (B) DND-41 is sensitive to JAK inhibition. SUPT-1, HPB-ALL, and DND-41 cells were treated with increasing concentrations of JAK inhibitor INCB018424. Cell proliferation was normalized to DMSO-treated cells. The average ± SEM of 3 repeats is shown. *Statistical significance. (C) IL-7R mutations identified in a sequencing screen in T-ALL patients (bold) and cell lines. (D) PTPRC nonsense and mis-sense variants identified in a sequencing screen in T-ALL patients (bold) and cell lines. (E) Western blot and quantitative PCR analysis of 9 T-ALL cell lines showing low CD45 expression and constitutive JAK1/STAT5 autophosphorylation in DND-41. PTPRC mRNA values are relative to hypoxanthine phosphoribosyl transferase and normalized to P12-Ichikawa. (F) Phosphatase activity of CD45 mis-sense variants. Beads coupled to wild-type and variant CD45 were incubated with increasing concentrations of DiFMUP, and initial rates of conversion to DiFMU were determined for each variant. The average ± SEM of 3 repeats is shown. Equal loading was assayed by Western blot analysis. D819V indicates phosphatase-dead CD45 mutant.