While well detectable tumor cell islands were observed in the tumor stroma of hematoxylin-eosin (H&E) stained mammary fat pads in both nude and NSG hosts at the MRD stage (Fig. 6B), the residual tumors in FVB/N mice showed high accumulation of stroma with only a few surviving tumor cells scattered as solitary cells within the tumor stroma, as evidenced by cytokeratin 14 (CK14) immunostaining (Fig. 6C). Here, KRT14 is linked to neoplasm.