Thus, the significant correlation (p = 0.001) between the expression of ABCG2 and of HER2 may suggest that ABCG2 is not only an ABC transporter which plays a role in drug-resistance of breast cancer chemotherapy but also a novel potential bio-marker which can predict biological behavior, clinical progression, prognosis and chemotherapy effectiveness. The gene discussed is ERBB2; the disease is breast carcinoma.