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.