To elucidate the roles of low-frequency CDMs in breast cancer, a sensitive and quantitative approach called ACB-PCR was employed to characterize the frequency and distribution of six CDMs between the four subtypes of breast cancer, including HR+/HER2+, HR+/HER2−, HR−/HER2+, and HR−/HER2− (TNBC). The gene discussed is HR; the disease is breast cancer.