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 are found in the human genome. It is estimated that L2 and L3 elements were active ~200-300 million years ago. Unlike L1 elements, L2 elements lack flanking target site duplications. The L2 (and L3) elements are in the same group as the CR1 clade, Jockey.
Long interspersed nuclear element
TRNA 
CccDNA
Gene
Replicon 
Cis-regulatory element
Insertion sequence
Exosome 