The genomes of several pathogens have been sequenced. This allows doctors to monitor the spread of diseases such as tuberculosis, identify resistant strains and identify proteins that could be useful drug targets. Sequencing pathogens from groups of people can show the pattern of transmission.Sequencing genomes can be used to identify phylogenetic relationships and how closely two species are related. In animals, cytochrome c oxidase varies between different species and the gene can be sequenced quickly and used in classification. In plants chloroplast DNA is used. This is known as barcoding. Scientists can work out the point of divergence from a putative common ancestor if they know the rate of mutation and build up theoretical evolutionary trees.