Similar to ER+ breast cancer cell line xenografts (Fig. 1, H to J), established ER+ luminal PDX tumors BCX1 and BCX2 grew significantly faster in the NSG-Pro mouse model compared to conventional NSG mice (Fig. 2, C and D), further demonstrating that the physiological levels of hPRL in the NSG-Pro model support the engraftment and the continuous growth of ER+ PDXs. Here, ESR1 is linked to breast cancer.