A method for identifying candidate genes that may determine significant differences between closely related species whose genomes have been mapped, e.g. between a pathogen and a closely related non-pathogenic bacterium. The genome of one species is displayed as a circle showing at points on its circumference the genes. The genes are assigned a colour code according to a priority of similarities between the species: closely homologous genes are selected first, then the less closely related, then those of the remaining genes that have other known functions (viral proteins, bacterium–host interaction factors) until finally the unknown candidate genes remain. The display may also show a smaller concentric circles which display G/C content or other potentially relevant information.