A four-gene signature (AR, FOXA1, SPDEF, TFF3) identified overlapping tumors with high sensitivity and specificity across datasets and remained accurate in RT-qPCR assays on FFPE material.<h4>Conclusion</h4>Luminal Androgen Receptor and Molecular Apocrine Breast Cancers constitute a single molecular entity within estrogen receptor-negative breast cancers. Here, SPDEF is linked to breast cancer.