The betagamma-crystallins form a superfamily of eye lens proteins comprised of multiple Greek motifs that are symmetrically organized into domains and higher assemblies. In the betaB2-crystallin dimer each polypeptide folds into two similar domains that are related to monomeric gamma-crystallin by domain swapping. The crystal structure of the circularly permuted two-domain betaB2 polypeptide shows that permutation converts intermolecular domain pairing into intramolecular pairing. However, the dimeric permuted protein is, in fact, half a native tetramer. This result shows how the sequential order of domains in multi-domain proteins can affect quaternary domain assembly.