After the successful engraftment of the human immune system, the mice were transplanted with the three breast cancer cell lines, JIMT-1 (HER2+/ER−), MDA-MB-231 (HER2−/ER−, TNBC) and MCF-7 (HER2−/ER+), mimicking different breast cancer subtypes. The gene discussed is ESR1; the disease is breast cancer.