Line1, also referred to as L1, are "non-LTR retrotransposons" that represent the greatest reshaping force of the human genome during evolution. They are transposable class I elements (retrotransposons) in the DNA of organisms. They belong to the group of long interspersed nuclear elements (LINEs).
Of the human genome, about 45% consists of transposable elements (long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and transposons); 5-8% are from viral sequences with similarity to infectious retroviruses. When retrovirus integration occurs in the germline, the integrated viral sequences are heritable (Katsura Y et al. 2019).
L1s make up approximately 17% of the human genome (Cervantes-Ayalc A et al. 2020).
The majority of L1s in the human genome are inactive; however, approximately 80-100 have retained the ability to retrotranspose, with significant variation between individuals. These active L1s can alter the genome through insertions, deletions, rearrangements, and copy number variations (CNVs).