Definitions for "Marker gene"
A gene with a clear-cut phenotype (and often a known location on a chromosome) that is used as a point of reference when mapping or selecting another gene at a nearby locus.
A gene or short sequence of DNA that acts as a tag for another, closely linked, gene.
Gene used during genetic engineering attempts that helps to identify cells that have received new DNA. Genes usually include either a selection advantage, e.g., antibiotic or herbicide resistance, or visualization advantage, e.g., beta glucuronidase (GUS) or green fluorescent protein (GFP) expression.