Mobile genetic elements in the pathogenesis of human reproductive disorders.

Mobile genetic elements (MGEs), or transposons, are autonomous DNA sequences capable of moving and proliferating within the genome. Long considered "selfish" or "junk" DNA, MGEs are now recognized as key components involved in genome evolution, the regulation of gene expression, and the pathogenesis of various diseases. In humans, MGEs are divided into two main classes: retrotransposons (Class I), which replicate via an RNA intermediate through a "copy-and-paste" mechanism, and DNA transposons (Class II), which move via a "cut-and-paste" mechanism without an RNA intermediate. According to the Human Genome Project, retrotransposons constitute the majority (approximately 42 %) of the MGE fraction within the human genome. The most abundant are the non-long terminal repeat (non-LTR) retrotransposons, dominated by autonomous LINE-1 elements. Although approximately 500,000 LINE-1 copies are present in the genome, the vast majority are defective, and only a small fraction (