The partial response rate was 44% (95% CI, 22% to 69%), meeting the primary end point. Responses were seen in patients with HER2 exon 20 insertions and point mutations in the kinase, transmembrane, and extracellular domains. Concurrent HER2 amplification was observed in two patients. HER2 immunohistochemistry ranged from 0 to 2+ and did not predict response, and responders had low HER2 protein expression measured by mass spectrometry. The median progression-free survival was 5 months (95% CI, 3 to 9 months).