KIF9 and male infertility: SPAG6 is crucial as a structural scaffold, and its deficiency causes male infertility and hydrocephalus.38 In C2-MOSP, mutations mainly affect SPAG16, KIF9, and CFAP20 (Fig. 7b), among which KIF9 mutations notably impair sperm motility.39 In large CA projections, such as C2a and C1b, mutations occur in CFAP65, CFAP70, SPEF2, and CFAP69 (Fig. 7b, c).