Many preclinical studies and clinical reports indicate that the addition of vitamin D (D2 or D3) or analogs of 1,25-Dihydroxycholecalciferol (1,25(OH)2D3, calcitriol) (Figure 1) to a breast cancer treatment regimen based on the use of selective estrogen receptor modulators (SERMs) or aromatase inhibitors (AIs) might improve the efficacy of chemotherapy and ameliorate musculoskeletal symptoms and joint pain in patients. Here, ESR1 is linked to breast cancer.