A complementary, potentially more quantitative technique is to directly enumerate the biodegradative gene(s) via quantitative polymerase chain reaction (QPCR), gene sequencing or functional gene microarrays. A prerequisite for genetic approaches, however, is that the involved genes can be clearly linked to a given transformation reaction. For instance, the atzD gene encoding cyanuric acid hydrolase correlates with atrazine biodegradation in agricultural soil surface layers, consistent with AtzD's cleavage of the s-triazine ring during bacterial atrazine metabolism.