Based on promising preclinical data, we conducted a single-arm phase II trial to assess the clinical benefit rate (CBR) of neratinib, defined as complete/partial response (CR/PR) or stable disease (SD) ≥24 weeks, in nonamplified metastatic breast cancer (MBC). Secondary endpoints included progression-free survival (PFS), toxicity, and circulating tumor DNA (ctDNA) detection. Tumor tissue positive for was required for eligibility. Neratinib was administered 240 mg daily with prophylactic loperamide. ctDNA sequencing was performed retrospectively for 54 patients (14 positive and 40 negative for tumor ). Nine of 381 tumors (2.4%) sequenced centrally harbored (lobular 7.8% vs. ductal 1.6%; = 0.026). Thirteen additional cases were identified locally. Twenty-one of these 22 cases were estrogen receptor positive. Sixteen patients median age 58 (31-74) years and three (2-10) prior metastatic regimens received neratinib. The CBR was 31% 90% confidence interval (CI), 13%-55%, including one CR, one PR, and three SD ≥24 weeks. Median PFS was 16 (90% CI, 8-31) weeks. Diarrhea (grade 2, 44%; grade 3, 25%) was the most common adverse event. Baseline ctDNA sequencing identified the same in 11 of 14 tumor-positive cases (sensitivity, 79%; 90% CI, 53%-94%) and correctly assigned 32 of 32 informative negative cases (specificity, 100%; 90% CI, 91%-100%). In addition, ctDNA variant allele frequency decreased in nine of 11 paired samples at week 4, followed by an increase upon progression. Neratinib is active in , nonamplified MBC. ctDNA sequencing offers a noninvasive strategy to identify patients with cancers for clinical trial participation. .