LINE1

[4] L1 activity has contributed to the instability and evolution of genomes and is tightly regulated in the germline by DNA methylation, histone modifications, and piRNA.

[5] L1s can further impact genome variation through mispairing and unequal crossing over during meiosis due to its repetitive DNA sequences.

[8][9] 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,[10] while mouse L1s contain an internal ribosome entry site (IRES) upstream of each ORF.

[11] The 5' UTRs of mouse L1s contain a variable number of GC-rich tandemly repeated monomers of around 200 bp, followed by a short non-monomeric region.

[20][21] Furthermore, less invasive blood assays for L1 copy number or methylation levels are indicative of breast or bladder cancer progression and may serve as methods for early detection.

[29] Increased RNA levels of Alu, which requires L1 proteins, are associated with a form of age-related macular degeneration, a neurological disorder of the eyes.

[31] In 2021, a study proposed that L1 elements may be responsible for potential endogenisation of the SARS-CoV-2 genome in Huh7 mutant cancer cells,[32] which would possibly explain why some patients test PCR positive for SARS-CoV-2 even after clearance of the virus.