CFAP91 (cilia and flagella associated protein 91) is an essential scaffolder protein that localizes to radial spoke (RS) structures within the axoneme of motile cilia and sperm flagella 1. The protein functions primarily as a structural component of the radial spoke complex, specifically stabilizing RS2 and RS3 base proteins and their adjacent inner dynein arms 1. CFAP91 interacts with multiple axonemal proteins including CFAP57, CFAP61, CFAP251, and LRRC23, and serves as a scaffolder for RS3 assembly 23. The protein also functions as a calmodulin-associated and spoke-associated complex (CSC) component and physically interacts with WDR66 4. Mechanistically, CFAP91 regulates cilium and flagellum motility by maintaining the positioning and stability of radial spokes, which transduce regulatory signals from the central pair apparatus to dynein arms 1. Loss of CFAP91 disrupts central pair complex organization and results in severely impaired axonemal beating 43. Clinically, biallelic CFAP91 variants (MAATS1 gene) cause multiple morphological abnormalities of the flagella (MMAF), characterized by severe central pair and radial spoke defects, leading to asthenozoospermia and primary male infertility 4. CFAP91-deficient males exhibit reduced sperm motility and compromised male fertility 3.