The amount of a particular enzyme present in a cell or tissue changes according to the rates of its synthesis and degradation. Factors affecting the rate of synthesis include the level of induction or repression of the gene encoding the enzyme and also the rate of degradation of the mRNA produced from that gene. Many key enzymes at control points in metabolic pathways have particularly short-lived mRNAs and the rate of enzyme synthesis is thus readily controlled by factors that affect the rate of gene transcription. The rate of degradation of an enzyme is reflected in its half-life – the time taken for 50% of the protein to be degraded. Most enzymes that are important in metabolic regulation have short half-lives, and are termed labile enzymes.