,4,6, 7, 8, 9 In a phase 1 trial of larotrectinib, a selective tropomycin receptor kinase (TRK) inhibitor, in patients with relapsed or nonresponsive solid tumors or central nervous system tumors, or infantile fibrosarcoma that would otherwise require disfiguring surgery, 14 of 15 patients (93%; 95% CI, 68–100) patients with a TRK fusion had an objective response.7 The gene discussed is NTRK1; the disease is fibrosarcoma.