CCDC40 encodes a coiled-coil domain protein essential for primary ciliary function through its role in axonemal architecture and dynein arm assembly. CCDC40 functions as part of a molecular ruler complex with CCDC39, determining the precise 96-nanometer repeat length of axonemal structures 1. This heterodimer is required for assembly of inner dynein arm (IDA) complexes and the dynein regulatory complex, which are critical for ciliary beat regulation 2. Beyond motility control, CCDC40 acts as a structural scaffold supporting assembly of an extensive network exceeding 90 ciliary proteins; loss of CCDC39/CCDC40 function results in absent IDA heavy chains (DNAH1, DNAH6, DNAH7) and widespread ciliary disorganization 31. Biallelic CCDC40 mutations cause primary ciliary dyskinesia (PCD), characterized by chr17 respiratory infections, bronchiectasis, and infertility 4. Patients with CCDC40 mutations exhibit particularly severe lung disease with accelerated FEV1 decline (-1.11% predicted/year) compared to other PCD genotypes, reflecting both motility-dependent and motility-independent pathogenic mechanisms 56. The severity correlates with impaired multiciliated cell maintenance, activation of proteostasis pathways, and defective epithelial barrier function 3.