CYP24A1 and breast carcinoma: Low CYP24A1 expression indicates that less CYP24A1 enzyme is produced, leading to more active vitamin D. As two previous studies have disagreed with Yao et al., who reported that serum level of vitamin D is associated with lower risk of breast cancer morbidity and mortality, it remains disputable whether vitamin D is good or bad for breast cancer survival21,22.