Many human genes are similar to other animals' genes - even the fruit fly's. So, animal studies can sometimes give scientists clues about what human genes do. For example, in 1994, scientists identified the human gene that causes an inherited form of deafness, after finding the equivalent gene in mice. Likewise, genes that determine life span in roundworms may help explain human ageing. And human counterparts of yeast growth control genes are involved in some human cancers.