text: 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. LINEs contain an internal Pol II promoter to initiate transcription into mRNA, and encode one or two proteins, ORF1 and ORF2. The functional domains present within ORF1 vary greatly among LINEs, but often exhibit RNA/DNA binding activity. ORF2 is essential to successful retrotransposition, and encodes a protein with both reverse transcriptase and endonuclease activity. 
  LINEs are the most abundant transposable element within the human genome, with approximately 20.7% of the sequences identified as being derived from LINEs. The only active lineage of LINE found within humans belongs to the LINE-1 class, and is referred to as L1Hs. 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 insertions in the genome.