Since the transcription and translation of separate cotransfected genes is not strictly correlated, the reliability of product expression based on selection of the cotransfected marker gene can be very low. Further- more, even under high selection pressure, the genetic stability of the expressed product can not be assured in long term cultivations. The combination of product and marker genes on the same vector does not completely alleviate these complications. For these reasons, dicistronic genetic configurations were developed. The first dicistronic constructs used IRES elements of picornaviral origin or from encephalomyocarditis virus (EMCV) for cap-independent translation of the second cistron while the first cistron relied on classical cap-dependent translation-initiation (Pelletier and Sonnenberg, 1988; Kaufman et al., 1991) . Although the two IRES elements differ completely at the sequence level, their secondary structure is very similar and typical for such internal translation initiators. A large number of dicistronic product-marker configurations have since been developed for many different applications. Table 2 gives an overview of recent dicistronic expression vectors. Although genetic combinations used for dicistronic expression vary among different applications, EMCV and picornaviral IRES remained the most popular cap-independent translation-initiators for dicistronic configurations because these elements function in a wide variety of cell lines including the industrially relevant CHO and BHK cell lines (Borman et al., 1997) . However, recent reports of varying translation-initiation capabilities of IRES in different host cell environments and discovery of new IRES elements is stimulating new development to apply these IRES elements for multicistronic expression (Bernstein et al., 1997; Schumacher and Wirth, 1997) . Dicistronic genetic configurations which contain the marker gene in the second cistron enable autoselective expression in addition to simultaneous and coordinated gene expression. Resistance to the marker gene or expression of the reporter gene is only possible if all 5 -encoded genetic elements are intact. This intrinsic self-selective program was found to be very reliable, with nearly all of the resistant cells also expressing the desired product gene (Gurtu et al., 1996; Rees et al., 1996) . Furthermore, product-marker configurations can also be used for efficient generation and screening of high producing cell clones: IRES-based translation-initiation of the second cistron is usually less efficient compared to cap-dependent translation, and can be decreased further by loss-infunction mutations of the IRES or the marker gene itself. The overall lower translation efficiency or activity is then compensated under high selective pressure by integration of the dicistronic expression unit into chromosomal sites with high transcriptional activity (Kaufman et al., 1991; Gurtu et al., 1996; Rees et al., 1996) . Certainly, simultaneous expression of two genetic traits can also be achieved by gene fusions (Krömer et al., 1997 ; pTracer plasmids of Clontech) or recently developed splicing expression technology (Lucas et al., 1996;  Figure 1 ), but gene fusion strategies are limited in functional applications or may lead to fusion products with altered physiologic specificities, and splicing-based two-gene expression leads to unequal expression levels of both proteins. Only IRESbased dicistronic expression guarantees simultaneous and coordinated expression of both transgenes at comparable levels for multi-subunit proteins (for example antibodies) which enables genetic configurations for a wide variety of contemporary research and development applications which are also listed in Table 2 (Dirks et al., 1993; Dirks et al., 1994; Fussenegger et al., 1997a) . Furthermore, IRES-mediated expression systems can be extended beyond the dicistronic level to tri-or even quattrocistronic artificial eukaryotic operons (Fussenegger et al., 1997b; Fussenegger et al., 1998c) .