The FMR1 5' leader exhibits putative IRES-activity, but also contains a cyrptic promoter. (A) Dicistronic luciferase DNA constructs containing the multiple cloning site (MCS) from pRF, the 5' leader from the β-globin or FMR1, or the EMCV IRES inserted into the intercistronic region were transfected individually into C6 cells. Luciferase activity is shown as the ratio of Photinus luciferase to Renilla luciferase (P:R) and is normalized to the activity obtained from the control construct, pRF. A P:R ratio that is above that obtained from pRF indicates the presence of an IRES. (B) Dicistronic luciferase DNA constructs containing the MCS from pRF, the β-globin or FMR1 5' leader or the EMCV IRES with or without the SV40 promoter were transfected into C6 cells. The Photinus luciferase activity from each transfection is shown. Each experiment was performed in triplicate, n = 3.