The Genome Within: Transposable Elements Are Not Junk, They Are a Hidden Operating System
Half of the human genome consists of sequences that do not code for proteins. For decades, the largest fraction of this non-coding DNA, transposable elements, or TEs, was dismissed as genomic junk: parasitic DNA that replicated selfishly, accumulated mutations, and occasionally caused disease when it jumped into the wrong place.
A major review published June 11 in Cell argues that this picture is no longer defensible. TEs are not merely genomic parasites. They are a hidden regulatory layer, a parallel genome within the genome that controls gene expression through two distinct but interconnected mechanisms: what their DNA does, and what their RNA does.
The review, by Jessica Sook Yuin Ho (Icahn School of Medicine at Mount Sinai), Christopher H. Douse (Lund University), and Ivan Marazzi (UC Irvine), proposes a dual-modality framework that reframes the genome as a “TE-rich ecosystem” in which mobile genetic elements are integrated as cis-regulatory DNA elements, structural scaffolds, non-coding RNA genes, and even mini-genes that produce functional peptides.
The authors draw a clean conceptual line between two ways TEs regulate the genome:
TE DNA as regulatory infrastructure. Over evolutionary time, TE insertions have been repurposed as promoters, enhancers, insulators, and repressors, providing transcription factor binding sites that influence when and where nearby genes are expressed. They shape three-dimensional genome architecture, acting as boundary elements that define topologically associating domains (TADs). Individual TE loci, rather than entire TE families, are now known to play highly specific, locus-dependent regulatory roles, a resolution made possible by long-read sequencing technologies that can map individual TE insertions across the genome.
TE RNA as functional transcripts. TEs also produce RNA molecules that regulate gene expression in their own right. These include long non-coding RNAs derived from TE sequences, small RNAs that modulate gene silencing, and enhancer RNAs that emerge from TE-derived enhancer elements. Some TE transcripts serve as mini-genes, producing truncated peptides or creating fusion transcripts by splicing into nearby protein-coding genes, a phenomenon known as TE-gene chimeras.
The two modalities are not independent. TE DNA provides the regulatory infrastructure for nearby genes; TE RNA from those same elements can act in trans on distant genes, creating a layered regulatory network that spans the entire genome.
From Junk to Ecosystem
The review’s central argument is that the gene-centric view of the genome, in which TEs are peripheral noise, should be replaced by an ecosystem model in which TEs are integral components of gene regulatory networks.
“Transposable elements are not just relics or parasites,” the authors write. “They are pervasive integrated elements that serve as cis-regulatory DNA, structural scaffolds for chromatin architecture, and functional RNA transcripts. Their contributions to gene regulation, evolutionary innovation, and disease are fundamental, not incidental.”
The evidence for this view has accumulated from multiple directions:
- Placental evolution. Endogenous retroviruses, ancient TE insertions derived from retroviruses, provided the syncytin genes essential for forming the syncytiotrophoblast layer of the placenta. Without TEs, mammalian placentas would not exist.
- Species-specific regulation. TEs are a primary source of species-specific regulatory elements. Human-specific TE insertions have rewired gene expression networks in ways that distinguish humans from other primates.
- Immune system integration. TE-derived nucleic acids are recognized by innate immune sensors including cGAS-STING, MDA5, and RIG-I. The intersection between TE regulation and antiviral immunity is a rapidly growing area of research.
The Disease Connection
The same features that make TEs powerful drivers of evolution also make them dangerous when their regulation breaks down.
- Cancer. TE reactivation is a hallmark of many cancers. LINE-1 retrotransposition creates genomic instability, TE insertions can activate oncogenes, and TE RNA transcripts can trigger innate immune signaling pathways that promote inflammation. Mutant KRAS, one of the most common oncogenic drivers, has been linked to TE RNA induction and interferon responses.
- Autoimmunity. When the epigenetic mechanisms that normally silence TEs fail, TE RNA accumulates in the cytoplasm, triggering type I interferon responses that can drive autoimmune pathology.
- Neurodegeneration. TE dysregulation has been documented in ALS and other neurodegenerative diseases. Somatic LINE-1 retrotransposition creates genetic mosaicism in the brain, and its consequences for neuronal function and survival are only beginning to be understood.
A Framework for the Field
The review synthesizes findings from roughly 300 references into a unified framework. The authors argue that the field has moved through three phases: the “junk DNA” era, the “repeat element” era in which TEs were studied as families, and the current “single-locus” era in which long-read sequencing and functional epigenomics allow researchers to study individual TE insertions with locus-specific resolution.
This shift has practical implications. TEs are being explored as biomarkers in cancer and autoimmune disease, and as tools for gene therapy, their natural ability to insert into genomes could be harnessed for therapeutic gene delivery. Understanding TE regulation is also critical for interpreting the function of the approximately 98% of the human genome that does not code for proteins, which remains largely unmapped in terms of disease relevance.
“The genome,” the authors conclude, “is not a collection of genes punctuated by junk. It is a TE-rich ecosystem in which mobile elements are integrated components of the regulatory architecture, DNA and RNA, structural and functional, inherited and emergent.”
Source: Ho, J.S.Y., Douse, C.H. & Marazzi, I. (2026). “Transposable element DNA and RNA: Drivers of gene expression, evolution, and disease.” Cell, 189(12), 3513-3540. DOI: 10.1016/j.cell.2026.05.003.