A major strength of the NSG-Pro mouse as a host for breast cancer PDX-development and drug response testing is the rescue of prolactin receptor (PRLR) signaling, which modulated the major therapy-targeted ESR1 and ERBB2 pathways based on molecular profiling of ER+ PDX tumors grown in NSG-Pro versus NSG mice. The gene discussed is ERBB2; the disease is breast carcinoma.