Marker genes are genes that confer a readily detectable phenotype on cells, either in culture or in transgenic or chimeric organisms — for example genes encoding reporter enzymes (GFP, GUS, luciferase) that produce a visible or measurable signal, or markers conferring antibiotic or herbicide resistance that allow selective growth. In food biotechnology and crop development they identify successfully transformed cells during strain or plant construction, dramatically speeding selection. Their use in food organisms faces regulatory scrutiny: antibiotic-resistance markers in particular raise concerns about horizontal gene transfer, so developers increasingly use marker-free systems or excisable markers, and the presence of residual marker DNA must be characterised in regulatory dossiers.