Copper nanoparticles (CuNPs) significantly reduce KRT19 expression in breast cancer cell lines, with greater suppression in epithelial-like subtypes (e.g., MDA-MB-468, FC = 0.32 at IC50) compared with mesenchymal-like subtypes (e.g., MDA-MB-231, FC = 0.51). Here, KRT19 is linked to breast carcinoma.