A typical L1 element is approximately 6,000 base pairs (bp) long and consists of two non-overlapping open reading frames (ORFs) which are flanked by untranslated regions (UTRs) and target site duplications. In humans, ORF2 is thought to be translated by an unconventional termination/reinitiation mechanism, while mouse L1s contain an internal ribosome entry site (IRES) upstream of each ORF.