Mammalian origins of replication are scattered at about 100-kb intervals along each chromosomal DNA fiber. It is not known whether origins are genetically defined sequences analogous to those that control initiation of replication in microorganisms. Since the advent of recombinant DNA techniques, it has become possible to search for origins in defined chromosomal domains. A variety of methods have suggested that nascent chains initiate at preferred locations on the chromosome, but there is some disagreement as to how precisely defined these initiation sites are.