Transposable elements (TEs) appear to govern the transition from oocyte to embryo in mice, according to a study in Developmental Cell this week. The paper questions the idea that some of the numerous repetitive TEs present in the genome are just "junk."

Barbara B. Knowles, from the Jackson Laboratory at Bar Harbor, Maine, and her group showed that the maternal transcriptome in mouse eggs and very early cleavage embryos contains an unusually high level of TEs that act as promoters and first exons for numerous RNA molecules, revealing a role as stage-specific alternative promoters for a number of host genes.

"We realized that in one of the two forms [of a gene under study], there was a retrotransposon at this unique time [the egg/embryo transition] integrated in front of it," Knowles told The Scientist. The researchers found that the first 20 amino acids of the gene consisted of...

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