It was shown that the shuffling mechanism for transcribed genome components, which involves a template switch during the RNA reverse transcription using the L1 retroelement enzymatic machinery, is common in mammals. The occurrence frequency of the resulting chimeric retroelements in the genomes of rodents is twice as high as in the DNA of primates. Moreover, we proved that not only single but also double switches may occur in vivo, which result in the fusion of copies of three different transcripts. Many of the identified chimeras are transcribed in mammals. © 2005 Pleiades Publishing, Inc.