Genetic testing was negative for mutations in ABL, AKT 1/2, CBL, CBLB, FBXW7, FLT3, FMS, GATA1, IDH2, JAK1/2, KRAS, MET, MPL, NRAS, NTRK1, PAX5, and SOS1. The patient was started on HyperCVAD Part A and rituximab, but a bone marrow biopsy 14 d after starting treatment showed a hypercellular marrow (90%) with 50% residual disease with the immunophenotype CD10, CD19, CD22, CD34, CD79a, and TdT+, which was consistent with B-ALL. Here, JAK1 is linked to precursor B-cell acute lymphoblastic leukemia.