For molecular biology research different types of markers may be used based on the selection sought. These include:
*Positive or selection markers are selectable markers that confer selective advantage to the host organism. An example would be antibiotic resistance, which allows the host organism to survive antibiotic selection.
*Negative or counterselectable markers are selectable markers that eliminate or inhibit growth of the host organism upon selection. An example would be thymidine kinase, which makes the host sensitive to ganciclovir selection.
*Positive and negative selectable markers can serve as both a positive and a negative marker by conferring an advantage to the host under one condition, but inhibits growth under a different condition. An example would be an enzyme that can complement an auxotrophy (positive selection) and be able to convert a chemical to a toxic compound (negative selection).