To enable visualization, tracking, and isolation of fibroblasts, we established a transgenic mouse model of breast cancer with fibroblast-specific reporter genes: transgenic mice that express the fluorescent reporter YFP under the collagen-1α promoter (Col1a1-YFP) were crossed with MMTV-PyMT mice to create PyMT;Col1a1-YFP transgenic mice, in which all fibroblasts are fluorescently labeled (Raz et al., 2018). The gene discussed is COL1A1; the disease is breast carcinoma.