A genetic marker is an easily observed gene. When the gene is expressed, the phenotypic effects are very obvious. The gene is consequently used to identify an individual or a cell that carries it. It can also be used as a probe to mark a nucleus, chromosome or locus.

Genetic markers can include the production of various easily screened chemicals, or more obvious effects such as colors or different structures. Genetic markers can also be used as an identifier for genetic engineering. After the marker is linked to a gene, its subsequent presence in cells indicates a successful genetic transformation. Markers of this nature can include antibiotic resistance, fluorescent genes, or other characteristics that the non-transformed organism is lacking.

Genetic markers are a powerful tool. Because of speed and cost-effectiveness, they are still used as an efficient method of screening organisms and producing initial maps of the genetic material of organisms, even when many more precise molecular tools are available.