Squamous cells make up the outer layer of several organs, such as skin, tongue, and esophagus. This image shows a tumor from a patient with squamous cell carcinoma. The cancer cells overproduce a cancer-causing version of the protein p63, called ∆Np63α (stained brown in this image). High levels of ∆Np63α make cancer cells become more like stem cells, which results in aggressive tumors.