Embryonic cell precursors (green) of Barrett’s esophagus at the junction between the stomach and esophagus (red). Barrett's esophagus arrises not from mutant cells in the esophagus but rather a small group of previously overlooked cells in all adults that can rapidly expand to cancer precursors when the normal esophagus is damaged by acid. The red antibody is against keratin 6 expressed on junctional squamous cells, whereas the green antibody is to keratin 7 expressed in embryonic precursors of Barrett's that reside at the junction.