This was an exploratory study in which we hypothesized potential differential protein expression by case characteristics (e.g., CYP27A1 associated with favorable prognostic characteristics such as tumor grade), and heterogeneity in associations between 27HC and breast cancer risk by tumor protein expression (e.g., stronger association between 27HC and breast cancer among cases with CYP27A1-positive tumors). The gene discussed is CYP27A1; the disease is breast cancer.