Iris of Eye with Model of GAF Domain (IMAGE) University of Washington Caption Image of the iris of researcher Clemens Heikaus' eye with a model of a GAF domain imbedded in the pupil. A messenger molecule binds to the GAF domain to regulate an enzyme, PDE6, that is central to the way light hitting the retina is converted to signals to the brain. Credit Brad Clifton 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.