Experiments were carried out in duplicate using seriallyin water-diluted mRNAs with final concentrations of 5 nM. Reactions contained 4 ml of an RNA solution, 4.5 ml of rabbit reticulocyte lysate (RRL, Promega), 0.25-1 ml of 35 S methionine (Amersham, in vitro translation grade), 0.5 ml of 1 mM amino acids lacking methionine and were incubated for 60 min at 28 C. Samples were boiled for 3 min in 2Â Laemmli buffer and loaded onto 12% SDS polyacrylamide gels. Gels were dried and exposed to phosphoimager screens. Band intensity of 0-frame and À1 frameshift products was measured using a Molecular Imager FX and Quantity One software (Biorad). Frameshift percentages were calculated as the amount of À1 frameshift product divided by the sum of 0 and À1 frame products, corrected for the number of methionines (10 in the 0-frame product and 28 in the fusion product), multiplied by 100.