A
gene which has an easily identifiable
phenotype so that one can tell apart cells or individuals which have the
gene and those which do not have it. Such a gene can also be used as a probe to mark cell
nuclei or
chromosomes so that they can easily be isolated or identified from other nuclei or chromosomes later.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.