RNASEH1 encodes ribonuclease H1, an endonuclease that specifically degrades the RNA component of RNA-DNA hybrid structures. In the nucleus, the enzyme plays a role in RNA polymerase II transcription termination by degrading R-loop formation at G-rich pause sites downstream of polyadenylation sites. RNASEH1 is also essential for mitochondrial DNA maintenance; pathogenic variants in RNASEH1 are among approximately 20 nuclear genes linked to mitochondrial DNA maintenance defects (MDMDs), a group of autosomal recessive or dominant disorders characterized by impaired mtDNA synthesis, resulting in quantitative depletion or qualitative deletions of mitochondrial DNA 1. Clinical manifestations range from mild adult-onset ophthalmoplegia to severe infantile hepatic failure. Beyond mtDNA maintenance, RNASEH1 functions in resolving R-loops to prevent transcription-replication conflicts; overexpression of RNaseH1 rescues synthetic lethality induced by topoisomerase 1 inhibition in MYC-driven cancers 2, and suppresses genomic instability in BRCA2-deficient cells 3. A non-coding RNASEH1 variant (rs7607888) associates with type 1 diabetes in Colombian families, with significant gene-environment interactions involving HLA alleles 4. These findings establish RNASEH1 as a multi-functional enzyme critical for genome stability, organellar integrity, and immune homeostasis.