Researchers often use short pieces of DNA called Alu polymorphisms to determine whether various populations are related to one another. Alus have no known function, yet they copy and insert themselves at random throughout a person's genome. Because previously inserted Alus do not excise themselves, Alu patterns can be used as yardsticks to estimate how close two people--and, on average, two populations--are genetically. For example, an Alu polymorphism on chromosome 1 occurs in roughly 95 percent of sub-Saharan Africans who have been sampled, 75 percent of Europeans and northern Africans, and 60 percent of Asians, whereas a different Alu polymorphism on chromosome 7 is carried by approximately 5 percent of sub-Saharan Africans, 50 percent of Europeans and northern Africans, and 50 percent of Asians. Some individuals carry both polymorphisms. No single polymorphism can, by itself, distinguish all the members of one major human group from all the members of another group, but by analyzing hundreds of these polymorphisms, scientists can group individuals sampled from different locations on the basis of their genetic profiles.