Definition for Selectable marker

From Biology Forums Dictionary

1. A gene whose presence can allow organisms (such as bacteria) to grow under a certain set of conditions. For example, an antibiotic resistance gene is a selectable marker that allows bacteria to grow in the presence of the antibiotic.

2. A selectable marker is not actually a required element of a plasmid, but it makes it possible for us to maintain stocks of cells that contain the plasmid uniformly. Sometimes, carrying a plasmid puts a cell at a selective disadvantage compared to its plasmid-free neighbors, so the cells with plasmids grow more slowly. Cells that happen to "kick out" their plasmid during division may be "rewarded" by having a higher rate of growth, and so these plasmid-free (sometimes referred to as "cured") cells may take over a population. If a plasmid contains a gene that the cell needs to survive (for example, a gene encoding an enzyme that destroys an antibiotic), then cells that happen to kick out a plasmid are "punished" (by subsequent death) rather than "rewarded" (as in the previous scenario). That selective pressure helps to maintain a plasmid in a population.