Endocrine therapy was reported to affect oxysterol concentrations in breast cancer patients with higher levels of 5b6b-EC and 27-HC after 28 days of aromatase inhibitor treatment, lower levels of 24S-HC, 25-HC, and 7a-HC after tamoxifen treatment [44], and higher levels of 7-KC one to two years after tumor removal and endocrine therapy use [45]. The gene discussed is CYP19A1; the disease is breast cancer.