Traditionally the bi-cistronic vector system has been used to define potential IRES-like sequences from a variety of viral and cellular mRNAs, and is recognized as being the standard test for this function [27] . A bi-cistronic vector is comprised of a 59 and 39 cistron; translation of the 59 cistron being cap-dependent and translation of the 39 cistron regulated by the putative IRES-like sequence. Thus, if the 39 cistron is translated in addition to the 59 cistron then the sequence of interest is said to have IRES-like properties, as translation is initiating internally.