Idiopathic non-obstructive azoospermia and cryptozoospermia are severe forms of male infertility in which sperm production is absent or extremely low and the cause is often unknown. This retrospective observational study examined whether mitochondrial DNA variants, particularly the MT-ND1 m.3700G>A variant, are associated with impaired sperm production in Chinese men.

Existing clinical records and available biospecimens from affected men, eligible family members, and fertile controls were analyzed to assess familial inheritance patterns, the frequency of the variant, and its association with infertility phenotypes. No study-related treatment or intervention was provided to human participants.

Inclusion Criteria:

• Men with idiopathic non-obstructive azoospermia or cryptozoospermia.
• Male patients undergoing a clinically indicated testicular biopsy, testicular sperm aspiration (TESA), microdissection testicular sperm extraction (micro-TESE), or a related clinical procedure, when residual clinical specimens are available.
• Comparison participants with normal spermatogenesis, including men with obstructive azoospermia and men undergoing sperm retrieval or testicular tissue evaluation for clinical reasons.
• Selected relatives and spouses of enrolled patients, when needed for genetic segregation analysis and determination of variant origin.

Exclusion Criteria:

• For the idiopathic non-obstructive azoospermia or cryptozoospermia cohort, azoospermia with an established alternative cause, including chromosomal abnormalities, Y-chromosome microdeletions, testicular tumors, severe trauma, prior radiotherapy or chemotherapy, or confirmed infection.
• Incomplete clinical data or inability to obtain informed consent.
• Biospecimens that do not meet quality requirements for the planned analyses.

Participants were selected from men receiving care at the Reproductive Medicine Center of the Third Affiliated Hospital of Guangzhou Medical University. The study population included men with idiopathic non-obstructive azoospermia or cryptozoospermia identified through clinical records and available biospecimens; men with normal spermatogenic function or obstructive azoospermia who served as comparison participants; and, when needed, available first-degree relatives and spouses for family-based genetic analyses. Testicular tissue was obtained only from residual specimens following clinically indicated testicular biopsy, TESA, or micro-TESE procedures.