Expression of DNA transposable elements during nervous system development

Transposable elements (retrotransposons and DNA transposons) comprise a large proportion of animal genomes, for example 20% in D. melanogaster, 36% in X. tropicalis and 45% in humans. After invading a new genome, the transposable element increases its copy number and subsequently accumulates mutations. These may eventually result in inactive copies. Until recent days transposons have been considered "junk" DNA and no clear function have been assigned for this important amount of information on genomes.