Gene rearrangements in a head-to-tail fashion have been described several times for gene segments of the rota-, phytoreo-, and orbiviruses. Several mechanisms have been proposed to explain the occurrence of partial duplications, however, none of these models has been fully satisfactory to explain the occurrence of all the observed duplicated genes. Based on recently available structural data about the lambda 3 RNA-dependent-RNA-polymerase of reoviruses, we propose the 'loop model' as a plausible explanation for the occurrence of partial gene duplications in dsRNA viruses. (c) 2005 Elsevier Inc. All rights reserved.