A selectable marker is a reporter gene introduced into a cell along with a gene insert. What this means is, the experimenter can tell the right gene is in the cell because the marker can be seen or detected.
Most often, this is used for bacteria or for cells in culture. Selectable markers show the success of a transfection or other procedure meant to introduce foreign DNA into a cell. It is a technique in gene targeting and gene knockout.
Selectable markers are often antibiotic resistance genes; bacteria that have been subjected to a procedure to introduce foreign DNA are grown on a medium containing an antibiotic. The antibiotic knocks out cells which do not have the resistant marker. Those bacterial colonies that can grow have successfully taken up and expressed the introduced genetic material.
An alternative to a selectable marker is a screenable marker, which allows the researcher to distinguish between wanted and unwanted cells.
Examples of selectable markers include: