Stopped at the border: boundaries and insulators

Boundaries in chromatin are often marked by the presence of insulator eleme nts. New results in Drosophila have identified an insulator with a proven b oundary function essential for development. Other studies suggest a connect ion between the activity of some insulators and Drosophila tritihorax-Group and Polycomb-Group genes. Several examples of vertebrate insulators have n ow been found; their locations suggest important boundary functions. Enhanc er-blocking studies in oocytes and position-effect studies in transformed c ells shed new light on insulator mechanisms.