In order to study the molecular mechanisms underlying DCIS progression, we developed a model that we refer to as mouse-intraductal (MIND). MIND involves injection of epithelial cells derived from patient DCIS into the mammary ducts of immunocompromised mice. DCIS MIND xenografts exhibited the full spectrum of human DCIS including invasive progression. Histology of xenografted DCIS lesions that progressed to invasion showed disruption of basement membrane and myoepithelial layer by the invasive cells, retraction of myoepithelial layer and microinvasion. Therefore, the DCIS MIND model is a valuable tool for studying the early molecular mechanisms underlying DCIS invasive progression in a manner that is individualized to each patient DCIS. This proposal that is aimed at further improving the translation application of MIND models by mimicking the natural microenvironment of human DCIS in mice, is directly responsive to this FOA. The following specific aims (SA) are proposed: SA 1) Humanize mouse mammary fat pads with patient derived immortalized fibroblasts and study effects on DCIS progression to invasion, pathology and biomarker expression. The DCIS xenografts humanized fat pads will be assessed for pathology, biomarker expression and progression to invasion. We expect the xenografted DCIS with humanized mammary fat pads to more closely resemble patient DCIS with respect to pathology and biomarker expression. We also expect the humanized fat pads to enhance DCIS invasive progression in a fraction of DCIS MIND xenografts. Additionally, we will correlate DCIS epithelial cell inherent molecular aberrations (gene expression and/or genomic aberrations) to DCIS invasive behavior in the DCIS MIND xenografts. SA 2) Reconstitute the mouse hematopoietic system with patient derived immune cells and study effects on DCIS progression to invasion, pathology and biomarker expression. The experimental procedure involves reconstitution of mouse hematopoietic system with patient derived immune cells. We will utilize MISTRG mice, which are highly permissive for human hematopoiesis including support of the development and function of monocytes, macrophages and NK cells. DCIS xenografts patient derived immune system will be assessed for human immune cell infiltration to DCIS, DCIS pathology and invasive behavior as described in SA 1. We will correlate the recruitment of specific immune cells to DCIS as well as DCIS epithelial cell inherent molecular aberrations (gene expression and/or genomic aberrations) to DCIS invasive behavior in the MIND xenografts.