Retrotransposons are jumping genes and they rule variation in organisms
Retrotransposition often fails to run its course, which produces truncated non-functional copies of the original L1 DNA.
Jumping genes called retrotransposons.
"Retrotransposons make up as much as half the nucleotides or DNA building blocks, in the human genome. In contrast, the approximately 25,000 protein-coding genes, we possess make up less than 2 percent of the mammalian DNA. The jumping genes are the descendants of the first primitive molecular replication systems that invaded the genomes of eukaryotes long ago."
"...were once thought of as nonfunctional junk DNA.
"What makes each Brain Unique?" Scientific American. Mach 2012, pp. 26-31