Genetic Variation in Human Gut Viruses (IMAGE) University of Pennsylvania School of Medicine Caption This is a phylogenetic tree of reverse transcriptase sequences. Credit Samuel Minot, Perelman School of Medicine, University of Pennsylvania, PNAS Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.