There is much current interest in the development of regulatable expression systems in basic functional genomic research, since externally regulated transcription enables the effects of a particular gene product Figure 3 . Tricistronic expression vectors enabling multigene metabolic engineering. Both vectors express the model product gene, the secreted alkaline phosphatase (SEAP) and one of the cell-cycle inhibitors p21 (pSS5) and p27 (pDD6). While the expression of p27 is sufficient to cause cell-cycle arrest and result in enhanced specific SEAP productivity which is additionally increased by coexpression of the anti-apoptosis gene bcl-x L , cell-cycle arrest using p21 is only effective when the differentiation factor c/ebpα is coexpressed and stabilizes p21.