Using tricistronic expression technology we extended the SEAP-p21-encoding dicistronic configu-ration by an additional cistron harbouring the differentiation factor, CCAAT/enhancer binding protein α (C/EBPα) (pSS5; Figure 3 ). C/EBPα has been shown to stabilise p21 at the protein level and also to induce endogenous p21 alleles (Timchenko et al., 1996) . Using this tricistronic set-up for metabolic engineering, the induction of conditional growth arrest of CHO cells was successful, and the sustained cell-cycle arrest achieved was accompanied by an up to 15-fold higher specific SEAP productivity compared to proliferationcompetent control cells, similar to that achieved by p27-based one-gene metabolic engineering (Fussenegger et al., 1998a and 1998c) .