The results of this univariate Cox regression showed that age, ethnicity, marital status, primary tumor location, molecular typing, PR status, ER status, whether radiotherapy, whether chemotherapy, T stage, N stage, M stage were the factors affecting the secondary thyroid cancer in breast cancer patients; while the results of multivariate Cox regression showed that age, race, whether radiotherapy, primary tumor location, N-stage, and M-stage were independent risk factors affecting the development of thyroid cancer in patients with breast cancer, and the results are shown in Table 4. Here, ESR1 is linked to breast cancer.