The Fluc versus the Rluc activities of the (-1) and (0) constructs were measured as relative light units with a Berthold Lumat LB 9507 luminometer, as previously described (8) . A Dual-Luciferase Reporter Assay System kit (Promega) was used for Jurkat cells and home-made reagents (25) were used for 293T cells. The Rluc activity is used to normalize the Fluc activity (Fluc/Rluc). The frameshift efficiency is equal to: ½FlucðÀ1Þ=RlucðÀ1Þ= ½Flucð0Þ=Rlucð0Þ þ FlucðÀ1Þ=RlucðÀ1Þ: