Despite the numerous expression vectors available containing dicistronic expression units ( Table 2) , most of these expression systems express a marker or reporter gene in a fixed configuration, leaving only one cistron free for heterologous gene expression. However, for one-step transfection of a product protein, metabolic engineering, and a selection marker in a single expression unit, multicistronic artificial mammalian operons with 3 or even 4 cistrons are desirable. We recently reported the construction of a novel vector family, pTRIDENT, for tricistronic gene expression in mammalian cells (Fussenegger et al., 1998c; Fig-ure 2) . A single promoter allows high level expression and, in some vectors, adjustable transcription of all three genes. Whereas the first cistron is translated in a classical cap-dependent manner, translation-initiation of the subsequent two cistrons rely on IRES elements of picornaviral (IRES; pTRIDENT1) and EMCV origin (denoted here CITE, cap-independent translation enhancer; third cistron; pTRIDENT3; Fussenegger et al., 1998c) .