A, Representative immunofluorescence staining of normal, tumor, and residual 3D organoids grown from primary mammary cells of a preclinical mouse model of breast cancer (Methods). Similar morphology of normal and residual organoids shown on the left with polarity markers ITGA6 (red), ZO-1 (green), GM-130 (magenta); DAPI (blue). Right: human MYC oncoprotein (green) is expressed only in tumor cells, CDH1 (red). Scale bar: 25 μm. Data information: Number of replicates corresponds to individual mice used to derive organoids. 