Multiple morphological abnormalities of the sperm flagella (MMAF) consistently lead to male infertility due to a reduced or absent sperm motility. Despite numerous genes described to be associated with MMAF, more than half of the cases analyzed remain unresolved, suggesting that many yet uncharacterized gene defects account for this phenotype. In this study, we identified six unrelated MMAF patients carrying a homozygous deleterious variant in MAATS1 encoding the CFAP91 protein.We also studied a flagellated protist, Trypanosoma brucei, and showed that the inactivation of TbCFAP91 in the parasite induced flagellar beating and ultrasctructural defects. Altogether our findings confirm that MAATS1 biallelic mutations lead to sperm motility impairment and primary male infertility. (By Prof. Charles COUTTON, https://jmg.bmj.com/content/early/2020/03/10/jmedgenet-2019-106775 )