RIBC1 is a microtubule inner protein localized to the axonemal A tubule inner sheath of sperm flagella 1. As a RIB43A domain-containing protein with coiled-coil regions, RIBC1 functions cooperatively with its homolog RIBC2 to regulate sperm motility 1. While loss of RIBC1 alone reduces sperm velocity, combined deletion of both RIBC1 and RIBC2 causes significantly decreased sperm motility and reduced fertility, with the double-knockout mice producing fewer pups than wild-type controls 1. Notably, these motility defects occur without detectable structural abnormalities in axonemal architecture at the ultrastructural level, suggesting RIBC1 and RIBC2 function in subtle regulatory mechanisms rather than gross flagellar organization 1. RIBC1 is absent from brain tissue and escapes X-inactivation in females 2, limiting its pathogenic potential in X-linked mental retardation despite mapping to the Xp11.22 duplication hotspot. The protein's primary physiological role is therefore restricted to male germ cell function, where it contributes to flagellar dynamics essential for reproductive success.