Translin is thought to participate in a variety of cellular activities including chromosomal translocations, translational regulation of m-RNA expression, and mRNA transport. It forms an octameric ring structure capable of sequence-specific binding of both DNA and RNA substrates. We have used electron microscopy and single-particle image analysis to generate a three-dimensional reconstruction of the Translin ring. The subunits appear to have two distinct domains that assemble to form an open channel with diameter of similar to 30 Angstrom at one end and similar to 50 Angstrom at the opposite end. In the presence of either DNA or RNA containing consensus binding sequences, the largest opening into the central cavity is filled with density. Strikingly, although Translin shows significant sequence homology to only one other protein, Translin-associated factor X, the quaternary organization and the dimerization of subunits in the ring are very similar to those observed for hexameric ring helicases. This suggests that many of the structures in DNA and RNA metabolism may have similar quaternary organization.