The products of many genes have several different effects or phenotypes in cells, a phenomenon known as pleiotropism or pleiotropy. Levels of pleiotropy vary from one gene to another, giving rise to expressions such as low-pleiotropy or great pleiotropy in the published scientific literature.

Genes encoding cytokines, polypeptide hormones used in cell-to-cell communication, may have many pleiotropic effects and, therefore, present great pleiotropy, as well as great redundancy because many functions of a given cytokine may also be performed by other different cytokines. For instance, several different cytokines may induce activated lymphocytes T (T cells) into proliferation. Examples of cytokines are as follows: Interferon alpha and beta (IFNalfa and IFNbeta), tumor necrosis factor (TNFalfa), interleukin-1 (IL-1alfa and IL-1beta).

Another example of great pleiotropy is given by GAPDH (enzyme glyceraldehyde-3-phosphate dehydrogenase). GAPDH was first known by its role in the metabolism of glucose, but several research...