Figure 1 : Bornavirus in the genome, for better or worse.
a, Horie et al.1 report that bornavirus gene sequences (red stars) became integrated into the germline of our ancestors, and through vertical transmission (by conventional inheritance) have become 'fixed' in the genome, thereby becoming endogenous viral insertions. A fixed viral insertion can follow one of two evolutionary fates: it can either decay into a pseudogene or be co-opted to form a new gene whose product has a cellular function. b, Circulating bornavirus sequences can become integrated into the genome of brain cells (the current target of Borna disease virus) after infection (exogenous insertion). These sequences are not heritable, but might cause mutations that interfere with brain function and may contribute to the development of psychiatric disorders.
