About 30%-40% of triple-negative breast cancers (TNBC) show HER2-low status and may benefit from the HER2-directed antibody-drug conjugate trastuzumab deruxtecan (T-DXd). Ataxia-telangiectasia mutated (ATM) kinase plays a crucial role in double-strand break (DSB) repair response, thus inhibitors of ATM, such as AZD-1390, may enhance the effect of DSB inducers, resulting in DNA damage accumulation.