Keapl is the substrate recognition module of a Cullin 3-based E3 ubiquitin ligase. Its primary role is to catalyze the ubiquitylation of the Nrf2 transcription factor. Oxidative stress blocks the E3 ligase activity of Keapl which stabilizes Nrf2 allowing it to drive the expression of certain antioxidant and drug metabolizing enzymes. A recent study identified a mutation in the Keapl gene (Keap1C23Y) that is present in breast cancer. Using reporter gene assays we show that Keap1C23Y is impaired in its ability to repress Nrf2 dependent transcription. Unlike wild-type Keapl, we found that Keap1C23Y failed to stimulate the degradation of Nrf2. Co-immunopreciptation experiments showed that Keap1C23Y retains its ability to interact with Nrf2 and Cullin 3. In contrast, we found that Keap1C23Y could not efficiently promote the ubiquitylation of Nrf2, suggesting that its intrinsic biological activity might have been compromised. These results revealed an unexpected role for the N-terminal region of Keapl in regulating its E3 ligase activity. Importantly, our findings suggest that a paradox exists whereby Nrf2 activity is beneficial in non-malignant cells but in cancer cells it may provide a selective advantage for clonal expansion. (c) 2007 Elsevier Inc. All rights reserved.