1 Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden. Electronic address: alexander.hayward@zoologi.su.se.
2 Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK. Electronic address: aris.katzourakis@zoo.ox.ac.uk.
Endogenous retroviruses are integral features of vertebrate genomes. Here, Hayward and Katzourakis offer a glimpse into the fascinating world of these mysterious partners in our evolution.