In a case-controls study of 579 women with primary incident breast cancer and 574 controls matched on age and time of blood collection, we examined serum concentrations of 25OHD at diagnosis or enrollment, with a particular focus on associations with breast cancer prognostic characteristics, specifically, tumor histologic grade, ER status, and molecular subtypes characterized by ER, PR and HER2. Here, ESR1 is linked to breast cancer.