At least 35% of the human genome is made up of transposon DNA. Retrotransposons are potential causal agents of human disease. The ‘mast’ human mobile element, L1 retrotransposon, has 5′,3′-UTR and two ORFs which encode a sequence-specific RNA-banding protein and a protein containing an endonuclease (EN) domain and a reverse transcriptase (RT) domain. It’s likely that L1 undergoes target-primed reverse transcription in order to carry out retrotransposon. The mobilization of the non-autonomous retrotransposons, such as Alu and processed pseudogens, require a cellular source of reverse transcriptase, which is most likely encoded by L1.