ESR1 and invasive breast carcinoma: Although tumor xenografts implanted in cleared mammary fat pads of immunocompromised mice with ER+/PR+ metastatic breast cancer cell lines, or patient-derived tumor specimens, have been used extensively to examine the role of ER and PR in progression of invasive breast cancer, this is not optimal as a DCIS xenograft model since the natural ductal microenvironment has been removed.