61Male infertility affects ~7% of men in Western societies, but its causes remain poorly 62 understood. The most clinically severe form of male infertility is non-obstructive azoospermia 63 (NOA), which is, in part, caused by an arrest at meiosis, but so far only few genes have been 64 reported to cause germ cell arrest in males. To address this gap, whole exome sequencing 65 was performed in 60 German men with complete meiotic arrest, and we identified in three 66 unrelated men the same homozygous frameshift variant c.676dup (p.Trp226LeufsTer4) in 67 M1AP, encoding meiosis 1 arresting protein. Then, with collaborators from the International 68Male Infertility Genomics Consortium (IMIGC), we screened a Dutch cohort comprising 99 69 infertile men and detected the same homozygous variant c.676dup in a man with 70 hypospermatogenesis predominantly displaying meiotic arrest. We also identified two 71Portuguese men with NOA carrying likely biallelic loss-of-function (LoF) and missense 72 variants in M1AP among men screened by the Genetics of Male Infertility Initiative (GEMINI). 73Moreover, we discovered a homozygous missense variant p.(Pro389Leu) in M1AP in a 74 consanguineous Turkish family comprising five infertile men. M1AP is predominantly 75 expressed in human and mouse spermatogonia up to secondary spermatocytes and 76 previous studies have shown that knockout male mice are infertile due to meiotic arrest. 77Collectively, these findings demonstrate that both LoF and missense M1AP variants that 78 impair its protein cause autosomal-recessive meiotic arrest, non-obstructive azoospermia 79 and male infertility. In view of the evidence from several independent groups and 80 populations, M1AP should be included in the growing list of validated NOA genes. 81 82We originally selected 64 azoospermic but otherwise healthy male patients who attended the 132 Centre of Reproductive Medicine and Andrology (CeRA), University Hospital Münster 133 (N = 51) or the Clinic for Urology, Pediatric Urology and Andrology, Gießen (N = 13), for 134 couple infertility. This is a subset of all patients included in our large-scale Male Reproductive 135 Genomics (MERGE) study, which currently comprises >800 men including 514 with NOA 136 ( Figure S1). All of the 64 patients were diagnosed with complete bilateral germ cell arrest at 137 the spermatocyte stage after evaluating at least 100 seminiferous tubules in tissue sections 138 of both testes accompanied by a negative TESE outcome, i.e., no sperm could be recovered.

The patient is a 27-year-old man with a constellation of symptoms including progressive, moderate, unilateral sensorineural hearing loss (SNHL) and tinnitus. Physical examination revealed a white patch of hair in the temporal region, one blue iris and one blue iris with a brown irridial patch that covered approximately 50% of the iris, no abnormalities of the medial canthii, and no pigmentary changes of the skin (Fig. 1). Audiograms done over a 5-year period showed progressive hearing loss during the previous 5 years on the left side. The head and neck examination was otherwise within normal limits, and the patient was healthy with a negative review of systems.