Refers to genes that can be used for species identifications. Different regions of DNA mutate at different speeds. Fast-changing regions are useful for population studies and paternity testing, while the most stable regions can be used for assessing deep evolutionary relationships between groups of organisms. Certain regions change at just the right rate to be stable within a species but different between species. These are known as barcode genes. The official barcode gene for animals is Cytochrome Oxidase 1 (COI or cox-1). Other genes used as animal barcodes include 12S, 16S, 18S and Cytochrome-b (cytb). For plants, the most commonly used genes are MatK, rbcL, trnL and ITS.