Methods
Ethical considerations and study participants consent
77 infertile male patients with unexplained (idiopathic) non-obstructive azoospermia or severe to extreme oligozoospermia and their unaffected parents were enrolled from the Radboudumc outpatient clinic and Newcastle upon Tyne Hospitals NHS Foundation Trust (Newcastle, UK). This study was approved by the ethics Committees/Institutional Review Boards ((Nijmegen: NL50495.091.14 version 5.0, Newcastle: REC Ref: 18/NE/0089). Written informed consent from all patients and their parents was obtained.