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the syncytins, which mediate fusion of trophoblasts dur- (HML2) family is considered potentially infectious, Sokol et al. Retrovirology (2015) 12:32 DOI 10.1186/s12977-015-0161-9moters or enhancers that may regulate adjacent human genes. In the human globin locus, an ERV9 modulates Department of Molecular Biology and Genetics, Aarhus University, Aarhus DK-8000, Denmarking placenta formation. Such diverse elements which are generally divided into human endogenous retroviruses and endogenous retrovirus-like elements (here collect-ively ERVs) comprise ~8 % of the human genome and originate from cumulative germ line infections and retrotranspositions in our ancestors (reviewed in [1,2]). While the genomes of some mammals such as mouse however, as functional viral proteins are encoded that produce non-infectious particles in teratocarcinomas and melanomas. Moreover, transcription of the HERV-