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Background: Severe spermatogenic failure (SPGF) represents one of the most relevant causes of male infertility. This pathological condition can lead to extreme abnormalities in the seminal sperm count, such as severe oligozoospermia (SO) or non-obstructive azoospermia (NOA). Most cases of SPGF have an unknown aetiology, and it is known that this idiopathic form of male infertility represents a complex condition. In this study, we aimed to evaluate whether common genetic variation in TEX15, which encodes a key player in spermatogenesis, is involved in the susceptibility to idiopathic SPGF. Materials and Methods: We designed a genetic association study comprising a total of 727 SPGF cases (including 527 NOA and 200 SO) and 1,058 unaffected men from the Iberian Peninsula. Following a tagging strategy, three tag singlenucleotide polymorphisms (SNPs) of TEX15 (rs1362912, rs323342, and rs323346) were selected for genotyping using TaqMan probes. Case-control association tests were then performed by logistic regression models. In silico analyses were also carried out to shed light into the putative functional implications of the studied variants. Results: A significant increase in TEX15-rs1362912 minor allele frequency (MAF) was observed in the group of SO patients (MAF = 0.0842) compared to either the control cohort (MAF = 0.0468, OR = 1.90, p = 7.47E-03) or the NOA group (MAF = 0.0472, OR = 1.83, p = 1.23E-02). The genotype distribution of the SO population was also different from those of both control (p = 1.14E-02) and NOA groups (p = 4.33–02). The analysis of functional annotations of the human genome suggested that the effect of the SO-associated TEX15 variants is likely exerted by alteration of the binding affinity of crucial transcription factors for spermatogenesis. Conclusion: Our results suggest that common variation in TEX15 is involved in the genetic predisposition to SO, thus supporting the notion of idiopathic SPGF as a complex trait., Spanish Ministry of Science and Innovation through the Spanish National Plan for Scientific and Technical Research and Innovation, Andalusian Government PID 2020-120157RB-I 00, Ministry of Science and Innovation, Spain (MICINN) Spanish Government PY20_00212 B-CTS-584-UGR20 MCIN/AEI IJC 2018-03802 6-I, European Commission FPU20/02926, Portuguese Foundation for Science and Technology, European Social Fund (ESF), National Funds, Portuguese Foundation for Science and Technology European Commission PEstC/SAU/LA0003/2013 POCI-01-0145-FEDER-007274 Portuguese State Budget of the Ministry for Science, Technology and High Education SFRH/BPD/120777/201 6 UID/BIM/00 009/2016 UIDB/00009/20 20, ToxOmics-Centre for Toxicogenomics and Human Health, Genetics, Oncology and Human Toxicology, Nova Medical School, Lisbon, Instituto de Salud Carlos III European Commission FEDER funds/European Regional Development Fund (ERDF) DTS18/001 01, SNS-Dpt, Generalitat de Catalunya, SNS-Dpt. Salut Generalitat de Catalunya 2017SGR191 Exp. CES09/020