The cover features a histology section from a patient's biopsy specimen of Barrett's esophagus stained with hemotoxylin and eosin (courtesy of Dr. Amitabh Srivastava, Dartmouth-Hitchcock Medical Center). The specimen features a region of dysplasia (upper left) next to a region of non-dysplasia, visually illustrating the concept of the phenotypic diversity that may be found within Barrett's esophagus. New work has provided a comprehensive analysis of measures of genetic diversity within a Barrett's esophagus segment as predictors of the risk for progression to esophageal adenocarcinoma. See articles by Merlo et al. (beginning on page 1388 ) and Michor and Polyak (beginning on page 1361 ) for more information.