Marker genes are tools used in gene transfer studies to determine if a transgene has been successfully transferred to an organism. There are two main types of marker genes: selectable marker genes and reporter genes. Selectable marker genes allow transformed cells to survive in toxic conditions by conferring resistance to antibiotics, herbicides, or antimetabolites. Common selectable markers confer resistance to kanamycin (nptII gene), methotrexate (dhfr gene), and glyphosate (epsps/aroA genes). Reporter genes can be quantified to assess transformation through the expression of proteins like β-glucuronidase (GUS gene) or green fluorescent protein (GFP gene), or through bioluminescence