Genes are regulated so that they are expressed only when the product is needed, since expression draws on limited resources.:7 A cell regulates its gene expression depending on its external environment (e.g. <answer>available nutrients, temperature and other stresses</answer>), its internal environment (e.g. cell division cycle, metabolism, infection status), and its specific role if in a multicellular organism. Gene expression can be regulated at any step: from transcriptional initiation, to RNA processing, to post-translational modification of the protein. The regulation of lactose metabolism genes in E. coli (lac operon) was the first such mechanism to be described in 1961.
What are examples of a cell's external environment?
available nutrients, temperature and other stresses