neering: Renner et al., 1995; Lee et al., 1996; Rivard et al., 1996; Greulich and Erikson, 1998) . Most of these successes have been realised by the addition of a single gene to the host cell's genome. However, just as single-gene interpretations of human disease have limited scope (Lander and Schork, 1994) , one-gene metabolic engineering cultured cells cannot access anything approaching the full potential set of useful engineered phenotypes (Papoutsakis, 1998) .