Gaining protection against future viral infection from a mobile genetic element

Retrotransposon is one type of mobile genetic elements that abundantly reside in the genomes of nearly all animals. Its uncontrolled activation is linked to sterility, cancer, and other pathological conditions, thereby being largely considered detrimental. Here we report within a strictly defined time window of development, retrotransposons activation can license the host's immune system for future antiviral responses.