The present invention provides, for the first time, the finding that the manganese superoxide dismutase Val16Ala polymorphism is significantly associated with prognosis for cancer patients treated with chemotherapeutic drug therapy. The alanine allele is a novel biomarker that predicts poor response and poor outcome to chemotherapeutic drug cancer therapy. Conversely, the valine allele predicts a good response and a good outcome to chemotherapeutic drug cancer therapy. Therefore, a genotype assay can be used to determine which alleles a subject is carrying, and subsequently this information can be used to determine if chemotherapeutic drug therapy is appropriate, and to customize therapy according to the patient's MnSOD genotype.