In this report we tested U-359, a novel synthetic 5-methylidenedihydrouracil analog which significantly inhibited proliferation in MCF-7 and MCF-7/Tx cells, as potential expression modulator of the ABCB1, ABCG2 and/or NF-κB proteins, considered for anticancer drug insensitivity in breast cancer cells. The gene discussed is ABCG2; the disease is breast carcinoma.