We observed a significant enrichment of HDR pathway members (including up-regulation of BRCA1, BRCA2, RAD50, and RAD51AP1) that were DE in BCX1 tumors grown in NSG-Pro mice compared to conventional NSG mice (FDR-adjusted P < 0.05; table S3), suggesting that the hPRL/PRLR signaling axis modulates HDR phenotypes in ER+ breast cancer PDX. Here, BRCA1 is linked to breast cancer.