Investigation of a new RNA-modification-based mechanism in mammalian spermatogenesis. We created a mice line devoid of testis-specific m5C RNA methyltransferase Nsun7. These mice demonstrated male infertility due to a peculiar defect in sperm tail assembly – mispositioning of the longitudinal columns of the fibrous sheath. The whole testis proteomes of wild type and Nsun7 knockout mice were obtained to study the molecular pathways affected by NSUN7.