Specific genes can be expressed in specific tissues (Schroeder et al., 2013), for example, syncytin-1 is highly expressed and tissue specific to the placenta (Mi et al., 2000; Matousková et al., 2006; Muir et al., 2006) however aberrant hypomethylation and expression of the syncytin-1 gene in the brain has been proposed to contribute to multiple sclerosis (Perron et al., 1997; Mattson and Taub, 2004). Here, ERVW-1 is linked to multiple sclerosis.