Nucleotide sequence and evolution of ETn elements.

The ETn (for "early transposon") family of long repeated sequences in abundantly transcribed in early mouse embryos from retroviral-like long terminal repeats. Nucleotide sequencing of two elements does not reveal any long open reading frame nor significant homology to retroviral proteins. The genetic polymorphism, monitored by Southern blotting within and across mouse species, reflects a concerted mode of evolution for the ETn sequences.