They are a type of reporter gene used in laboratory microbiology, molecular biology, and genetic engineering to indicate the success of a transfection or transformation or other procedure meant to introduce foreign DNA into a cell.
The genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline, kanamycin, etc., are all widely used as selectable markers for molecular cloning and other genetic engineering techniques in E. coli.
Selectable markers allow scientists to separate non-recombinant organisms (those which do not contain the selectable marker) from recombinant organisms (those which do); that is, a recombinant DNA molecule such as a plasmid expression vector is introduced into bacterial cells, and some bacteria are successfully transformed while some remain non-transformed.
An alternative to a selectable marker is a screenable marker, another type of reporter gene which allows the researcher to distinguish between wanted and unwanted cells or colonies, such as between blue and white colonies in blue–white screening.
[citation needed] For molecular biology research, different types of markers may be used based on the selection sought.