HERVs as regulators of host gene expression. HERV LTRs can influence expression of flanking host genes (host exons are indicated by black boxes and HERV is indicated in grey). (a) The normal transcript from the cellular promoter. (b) An alternative transcript from the HERV LTR (for example, see ). Either the 5' or the 3' LTR can be used, but transcripts from the 5' LTR may use the HERV splice donor site and could potentially generate alternative transcripts. Expression of the cellular gene may be either activated or silenced by this mechanism. (c) The activity of the HERV LTR may be modulated by other factors such as methylation , polymorphisms  and the activation state of the cell.