One study found a lower risk of breast cancer recurrence in those who were supplemented with vitamin D post-diagnosis, but only among estrogen receptor (ER)-positive tumors and not among ER-negative tumors (HR = 0.64, 95% CI: 0.47–0.87 and HR = 1.25, 95% CI: 0.78–1.98; respectively) [35]. This evidence concerns the gene ESR1 and breast cancer.