NADPH oxidase 1 (NOX1) is highly expressed in colon epithelial cells, where it generates reactive oxygen species (ROS) to interact with normal and pathogenic bacteria. Excessive reactive ROS production is associated with damage to the intestinal mucosa, particularly in mucosal lesions of inflammatory bowel disease (IBD). Studies have shown that NOX1 levels are increased in human prostate cancer, and might also play a role in angiogenesis, cell growth, and tumor pathogenesis. The identification of potent, selective inhibitors of NOX1 may lead to potential therapeutic candidates for excess cell proliferation, cancer, and IBD. This project demonstrated that the molecular probe ML090 (CID-616479) is neither a hydrogen peroxide scavenger, nor a general cell toxin on the time scale of cellular NOX inhibition assays. The specificity of the probe for NOX1 over NOX2, 3 and 4 in a 293 assay system suggests that a target specific to the NOX1 system is the molecular target. ML090 should serve as a useful probe for cellular systems where inhibition of NOX1, and not other members of the NOX family, is desired. This compound provides a significant improvement over the previously existing non-selective NOX inhibitor, diphenylene iodium.