Transposable elements in health and disease: Molecular basis and clinical implications.

Transposable elements (TEs), once considered genomic "junk", are now recognized as critical regulators of genome function and human disease. These mobile genetic elements-including retrotransposons (long interspersed nuclear elements [LINE-1], Alu, short interspersed nuclear element-variable numbers of tandem repeats-Alu [SVA], and human endogenous retrovirus [HERV]) and DNA transposons-are tightly regulated by multilayered mechanisms that operate from transcription through to genomic integration. Although typically silenced in somatic cells, TEs are transiently activated during key developmental stages-such as zygotic genome activation and cell fate determination-where they influence chromatin architecture, transcriptional networks, RNA processing, and innate immune responses. Dysregulation of TEs, however, can lead to genomic instability, chronic inflammation, and various pathologies, including cancer, neurodegeneration, and aging. Paradoxically, their reactivation also presents new opportunities for clinical applications, particularly as diagnostic biomarkers and therapeutic targets. Understanding the dual role of TEs-and balancing their contributions to normal development and disease-is essential for advancing novel therapies and precision medicine.