LINE-1: a mammalian transposable element.

Correspondence: T.G. Fanning, Laboratory of Biochemistry, National Cancer Institute, Bldg. 37, Rm. 4A01, Bethesda, MD 20892, U.S.A. domesticus and Homo sapiens, respectively. This taxonomic differentiation of Lls is not superfluous, since structural differences exist between Lls from different taxonomic orders, and significant differences may even exist between Lls found in different genera within an order. L1 elements appear to belong to a subclass of retrotransposons (class 11) and to amplify by a process that includes an RNA intermediate. We begin by describing a consensus L1; that is, those attributes that are thought to be characteristic of all known Lls. We will then describe the differences that characterize specific L1 elements within a species, as well as general differences that appear to exist between different taxonomic