Neoadjuvant chemotherapy is a standard therapy for patients with locally advanced breast cancer (LABC) and is increasingly used for early stage operable breast cancer. Not all patients benefit from it, and reliable markers for predicting response are needed. The cytotoxic effects of chemotherapy are mediated by induction of DNA damage in tumor cells. There is evidence that resistance to chemotherapy is related to enhanced repair of DNA lesions. The postreplication DNA repair (PRR) or translesion synthesis backup DNA repair pathway is critical for cell viability, conferring tolerance to DNA damaging drugs, and maintenance of genomic integrity. However, despite its importance in conferring tolerance to a variety of DNA damaging drugs including cytotoxic chemotherapy, the involvement of this backup repair pathway in chemotherapy response has not been studied. The Rad6B protein is a fundamental component of PRR. We have shown previously that the ability of breast cells to tolerate chemotherapeutic drugs correlates with Rad6B expression levels and PRR capacity. To determine whether Rad6B expression/distribution can be used singly or in combination with p53, Mdr-1/PgP, PCNA or beta-catenin as predictors of response to neoadjuvant chemotherapy, we analyzed posttreatment samples from 20 patients with LABC in a retrospective study. Only preferential Rad6B nuclear localization was associated with response to neoadjuvant chemotherapy. Nuclear exclusion with cytoplasmic overexpression of Rad6B was observed in some patients who failed to respond, but the association with response is not statistically significant. This is the first study to report that the postreplication DNA repair protein Rad6B could be used as an independent marker for determining response to neoadjuvant chemotherapy. This is an exploratory study and larger studies utilizing interim evaluations of Rad6B expression, its subcellular localization and repair activity are required to confirm its utility as a predictor of chemotherapeutic response.