All of the PDXs used in this study were originally derived by transplantation of a fresh treatment-naïve patient breast tumor biopsy into the cleared mammary gland fat pad of immunocompromised mice [20]; they are triple negative (i.e., ER−, PR−, and HER2−) with positive expression of both EGFR and HER3, with the exception of BCM-4664, which displays low EGFR expression and HER3− (Additional file Tables: Table S1); additional information includes, whenever available, TP53 mutations, PAM50, and Pietenpol subtype classification (Additional file Tables: Table S1) [20, 32–34]. This evidence concerns the gene ESR1 and breast neoplasm.