Larotrectinib, a highly selective TRK inhibitor, demonstrated significant and durable antitumor activity in patients with TRK fusion-positive cancers across all age groups and tumor types. In a study involving 55 patients with TRK fusion-positive cancers, the overall response rate was 75% according to independent review and 80% according to investigator assessment. At one year, 71% of responses were ongoing, and 55% of patients remained progression-free. The median duration of response and progression-free survival had not been reached. Most adverse events were grade 1, and no grade 3 or 4 events were considered related to larotrectinib. No patient discontinued treatment due to adverse events. TRK fusions are associated with oncogene addiction and may occur in up to 1% of solid tumors. Larotrectinib was effective in various tumor types, including mammary analogue secretory carcinoma, infantile fibrosarcoma, thyroid tumor, colon tumor, lung tumor, melanoma, and gastrointestinal stromal tumor. The drug was well tolerated, with few severe adverse events. Resistance mechanisms included mutations in the kinase domain of TRK, such as solvent front substitutions, which reduced the drug's inhibitory potency. Acquired resistance was observed in some patients, but many continued treatment due to clinical benefit. Larotrectinib showed rapid, potent, and durable antitumor activity in children and adults with TRK fusion-positive cancers. The drug's safety profile suggests long-term administration is feasible. Screening for TRK fusions is essential to identify patients who may benefit from larotrectinib. Additional data will be needed to further evaluate the drug's safety and long-term efficacy. The study highlights the importance of TRK fusions as therapeutic targets and the potential for larotrectinib to be effective in a wide range of cancers.Larotrectinib, a highly selective TRK inhibitor, demonstrated significant and durable antitumor activity in patients with TRK fusion-positive cancers across all age groups and tumor types. In a study involving 55 patients with TRK fusion-positive cancers, the overall response rate was 75% according to independent review and 80% according to investigator assessment. At one year, 71% of responses were ongoing, and 55% of patients remained progression-free. The median duration of response and progression-free survival had not been reached. Most adverse events were grade 1, and no grade 3 or 4 events were considered related to larotrectinib. No patient discontinued treatment due to adverse events. TRK fusions are associated with oncogene addiction and may occur in up to 1% of solid tumors. Larotrectinib was effective in various tumor types, including mammary analogue secretory carcinoma, infantile fibrosarcoma, thyroid tumor, colon tumor, lung tumor, melanoma, and gastrointestinal stromal tumor. The drug was well tolerated, with few severe adverse events. Resistance mechanisms included mutations in the kinase domain of TRK, such as solvent front substitutions, which reduced the drug's inhibitory potency. Acquired resistance was observed in some patients, but many continued treatment due to clinical benefit. Larotrectinib showed rapid, potent, and durable antitumor activity in children and adults with TRK fusion-positive cancers. The drug's safety profile suggests long-term administration is feasible. Screening for TRK fusions is essential to identify patients who may benefit from larotrectinib. Additional data will be needed to further evaluate the drug's safety and long-term efficacy. The study highlights the importance of TRK fusions as therapeutic targets and the potential for larotrectinib to be effective in a wide range of cancers.