Based on the possible relationship between vitamin D levels and arthralgia, several clinical trials have been conducted by supplementing patients on aromatase inhibitor therapy with vitamin D. All these trials include women with hormone receptor-positive breast cancer to be treated with an aromatase inhibitor and had low blood levels of vitamin D. Patients given with a supplement of 400 IU vitamin D once a week for 8 weeks, and once a month until the end of the study, showed an improvement in pain, especially those patients with lower vitamin D levels at the beginning of the study [126]. The gene discussed is CYP19A1; the disease is breast carcinoma.