Downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation.
An example of downregulation is the cellular decrease in the number of receptors to a molecule, such as a hormone or neurotransmitter, which reduces the cell's sensitivity to the molecule. This phenomenon is an example of a locally acting negative feedback mechanism.
An example of upregulation is the increased number of cytochrome P450 enzymes in liver cells when xenobiotic molecules such as dioxin are administered (resulting in greater degradation of these molecules) or the increased numbers of NMDA glutamate receptors found in people who have consumed excessive quantities of alcohol (thereby inhibiting those same receptors).
Upregulation and downregulation can also happen as a response to toxins or hormones, for example in pregnancy, hormones cause cells in the uterus to become more sensitive to oxytocin.