We have shown previously by Southern blot analysis that Bov-B long interspersed nuclear elements (LINEs) are present in different Viperidae snake species. To address the question as to whether Bov-B LINEs really have been transmitted horizontally between vertebrate classes, the analysis has been extended to a larger number of vertebrate, invertebrate, and plant species. In this paper, the evolutionary origin of Bov-B LINEs is shown unequivocally to be in Squamata Long interspersed nuclear elements (LINEs) or non-long terminal repeat retrotransposons are interspersed in all eukaryotic genomes so far studied and appear to include a few active elements and a large number of 5Ј truncated and nonfunctional copies (1). These elements have quite a narrow distribution and are mostly genus-, family-, order-, or classspecific, like LINE-1 elements in mammals (2). The only example of a LINE family with phylum-wide distribution is the vertebrate CR1-LINEs (3). The mammalian LINE-1 (4) and arthropod R1 and R2 elements (5, 6) are stable components and have been maintained by vertical transmission in their genomes throughout the entire history of mammalian and arthropod lineages. Recently, several reports raised the