We found that co-treatment with DHA enhanced the drug sensitization of doxorubicin in already doxorubicin-resistant MCF-7 breast cancer cells by inhibiting cell proliferation and invasion, inducing G2/M phase cell cycle arrest, enhancing drug accumulation in cancer cells, inducing the expression of apoptotic genes, inducing lipid peroxidation, and down-regulating drug efflux-regulating genes such as P-gp and TG2. The gene discussed is PGP; the disease is cancer.