Selectable markers are genes that allow transformed cells expressing them to be selected against large populations of non-transformed cells. In recombinant DNA research, the marker gene is physically linked to the gene of interest. Both genes have their independent regulatory elements like promoters, terminators etc. Depending upon the host, the marker genes can code for herbicide or antibiotic resistance. The use of antibiotic resistance genes as markers is well practiced in microbial genetics as a laboratory tool. Now, a similar concept has been extended to the genetic transformation of other organisms, including agricultural crops. For the expression of selectable markers in plants, genes are often chemically synthesized with codons optimized for expression in plants.