Within a single bacterial cell, genes are reversibly induced and repressed by transcriptional control in order to adjust the cell’s enzymatic machinery to its immediate nutritional and physical environment. Single-celled eukaryotes, such as yeasts, also possess many genes that are controlled in response to environmental variables (e.g., nutritional status, oxygen tension, and temperature). Even in the organs of higher animals — for example, the mammalian liver — some genes can respond reversibly to external stimuli such as noxious chemicals. In general, however, metazoan cells are protected from immediate outside influences; that is, most cells in metazoans experience a fairly constant environment. Perhaps for this reason, genes that respond to environmental changes constitute a much smaller fraction of the total number of genes in multicellular organisms than in single-celled organisms.