Since 1993, Dr. Sherr's laboratory has conducted research on how common environmental pollutants, such as dioxins, polycyclic aromatic hydrocarbons (PAHs) and PCBs, suppress the immune system and induce cancer. This work has focused on molecular signals initiated by the aryl hydrocarbon receptor (AhR), a cell protein that famously is activated by a variety of common environmental chemicals. Reasoning that evolution did not select out the AhR to recognize human pollution, the Sherr lab began using environmental chemicals that activate the AhR to learn what the true function of this curious protein is. The Sherr lab was the first to demonstrate dramatically high levels of the AhR in breast cancer, an observation that would eventually be extended to cancers of the oral cavity, brain, and lung. The lab showed that, while the AhR in these cancers could be hyper-activated with environmental chemicals, the receptor was already in the “on” position, even in the absence of such chemicals. Furthermore, the Sherr lab determined that this chronic on signal was mediated at least by production of endogenous AhR activators (agonists) produced in excessive levels by malignant cells themselves. Using novel AhR inhibitors that the laboratory developed, and state-of-the-art molecular technologies including CRISPR/Cas9 gene editing, the lab demonstrated that pre-cancers and cancers were exploiting this receptor/signaling protein to up-regulate expression of genes that drive cancer invasion, metastasis, and longevity and, in doing so, enhance the production of "cancer stem cells", a small subset of the tumor capable of withstanding chemo- and radiation therapy.