Eligible patients with HER2 exon-20 insertion mutations confirmed by next-generation sequencing or multiplex polymerase chain reaction platforms and a history of one or two lines of chemotherapy received T-DM1 (3.6 mg/kg) intravenously every 21 days….A775_G776insYVMA was the most frequent HER2 exon-20 insertion mutation, accounting for 19 (86.4%) of the 22 patients. The ORR was 38.1% (90% confidence interval, 23.0-55.9%), and the disease control rate was 52.4%. The median duration of response was 3.5 months, and the median progression-free survival and median overall survival were 2.8 and 8.1 months, respectively.