Previous research has shown that various risk factors affect breast cancer differently depending on the estrogen level,[29] so this study combined two-sample MR and multiple-sample MR analysis to investigate whether there is a causal association between 25(OH)D levels and the outcome of breast cancer with varying estrogen receptor (ER) status, to evaluate its impact, and to develop a more individualized treatment for patients. The gene discussed is ESR1; the disease is breast cancer.