The biological information fundamental to life is encoded in the DNA molecule. The DNA regions that encode proteins are called genes. The degree to which a gene is expressed in the phenotype is called penetrance. Gene expression is the extent to which the penetrable gene is expressed in an individual. Expression and penetrance depend on genotype and environment. When we say that gene is expressed, we mean that the expression of information which it possesses, i.e. the formation of proteins. Enzymes are highly specialized proteins with catalytic power. Because of that they are crucial to every biochemical process. Each cell, depending on its function in the body, has a different gene expression. The expression of genes in an organism can be influenced by the environment, including the external world in which the organism is located or developing, as well as the organism's internal world, which includes such factors as hormones or metabolism. Oxygen is essential for the development and growth of multicellular organisms. Organisms must therefore be adapted to hypoxia, to preserve their biological functions and continue proper growth and development. In mammals, HIF-1 factor ("hypoxia-inducible factor") is a main oxygen control regulator during embryonic and postnatal development of the organism. HIF-1 has two subunits, α and β. Subunit regulation depends on hydroxylation of prolyl and asparagil acid residues. In plants under hypoxia condition, a transition from aerobic metabolism to fermentation occurs, and there is a better translation of stress proteins. Organisms have developed a sophisticated physiological network to maintain oxygen homeostasis at the tissue level. One of the critical aspects of this network is the ability to sense and respond to low-oxygen conditions. Adaptation to hypoxia at the organism level includes regulation of many proteins at the level of gene expression. Temperature is an important environmental factor that affects gene expression. In mammals temperature have a visible effect on phenotype (coat colour of Siamese cats), and in many reptiles sex determination is temperature dependent. Expression of certain genes and synthesis of specific proteins are common to both heat and cold stress in plants. The difference is that during the stress caused by coldness the expression of common ("housekeeping") proteins is not reduced.