A cell’s genotype represents the cell’s genetic potential, whereas its phenotype represents the expression of a culture’s potential. The genotype of a cell can be altered by mutations. Mutations may be selectable or unselectable . The rate of mutation can be enhanced by the addition of chemicals called mutagens or by radiation. Auxotrophs are of particular use in genetic analysis and as a basis for some bioprocess. Another useful class of mutants is conditional mutants. Gene transfer from one cell to another augments genetic information in ways that are not possible through mutation only. Genetic recombination of different DNA molecules occurs within most cells. Thus, genetic information transferred from another organism may become a permanent part of the recipient cell. The three primary modes of gene transfer in bacteria are transformation, transduction, and conjugation . We can use gene transfer in conjunction with restriction enzymes and ligases to genetically engineer cells. In-vitro procedures to recombine isolated donor DNA gens with vector DNA (for example plasmids, temperate phages, or modified viruses) are called recombinant DNA techniques . The application of recombinant DNA technology at the commercial level requires a judicious choice of the proper host-vector system. E. coli , S. cerevisiae , P. stipitis and Bacillus are commonly selected as hosts because of their unique properties. Animal cell culture is required when posttranslational modifications are essential. The vector must be designed to optimize a desired process. One must be aware of the regulatory constraints on the release of cells with recombinant DNA. These are particularly relevant in plant design, where guidelines for physical containment must be met. Deliberate release of genetically modified cells is possible, but extensive documentation will be required. Two increasingly important applications of genetic engineering are metabolic or pathway engineering for the production or destruction of nonproteins and protein engineering for the production of novel or specifically modified proteins.