Spurious splicing within the XIAP 5′ UTR occurs in the Rluc/Fluc but not the βgal/CAT bicistronic reporter system

X-chromosome-linked inhibitor of apoptosis, XIAP, has been shown to contain a strong internal ribosome entry site (IRES) within its 5′ untranslated region (UTR) that promotes translation of XIAP mRNA under conditions of cellular stress. This claim came under scrutiny in a recent report demonstrating that the XIAP 5′ UTR undergoes splicing when inserted between the two reporter cistrons of the dual luciferase plasmid Rluc/Fluc. In this paper, we demonstrate that the splicing within the XIAP 5′ UTR specifically occurs only in the context of mRNA produced from the Rluc/Fluc but not the pβgal/CAT bicistronic reporter plasmid.