Long interspersed nuclear elements (LINEs) (also known as long interspersed nucleotide elements or long interspersed elements) are a group of non-LTR (long terminal repeat) retrotransposons that are widespread in the genome of many eukaryotes. They make up around 21.1% of the human genome. LINEs make up a family of transposons, where each LINE is about 7,000 base pairs long. LINEs are transcribed into mRNA and translated into protein that acts as a reverse transcriptase. The reverse transcriptase makes a DNA copy of the LINE RNA that can be integrated into the genome at a new site. The only abundant LINE in humans is LINE1. The human genome contains an estimated 100,000 truncated and 4,000 full-length LINE-1 elements. Due to the accumulation of random mutations, the sequence of many LINEs has degenerated to the extent that they are no longer transcribed or translated. Comparisons of LINE DNA sequences can be used to date transposon insertion in the genome. History of discovery The first description of an approximately 6.4 kb long LINE-derived sequence was published by J. Adams et al. in 1980. Types Based on structural features and the phylogeny of its key enzyme, the reverse transcriptase (RT), LINEs are grouped into five main groups, called L1, RTE, R2, I and Jockey, which can be subdivided into at least 28 clades. In plant genomes, so far only LINEs of the L1 and RTE clade have been reported. Whereas L1 elements diversify into several subclades, RTE-type LINEs are highly conserved, often constituting a single family. In fungi, Tad, L1, CRE, Deceiver and Inkcap-like elements have been identified, with Tad-like elements appearing exclusively in fungal genomes. All LINEs encode a least one protein, ORF2, which contains an RT and an endonuclease (EN) domain, either an N-terminal APE or a C-terminal RLE or rarely both. A ribonuclease H domain is occasionally present. Except for the evolutionary ancient R2 and RTE superfamilies, LINEs usually encode for another protein named ORF1, which may contain an Gag-knuckle, a L1-like RRM (), and/or an esterase. LINE elements are relatively rare compared to LTR-retrotransposons in plants, fungi or insects, but are dominant in vertebrates and especially in mammals, where they represent around 20% of the genome. L1 element The LINE-1/L1-element is one of the elements that are still active in the human genome today. It is found in all mammals except megabats. Other elements Remnants of L2 and L3 elements