DNAH9 (dynein axonemal heavy chain 9) is a force-generating motor protein essential for ciliary and flagellar motility. As a component of the outer dynein arm complex, DNAH9 generates force directed toward the minus ends of microtubules through ATPase activity, with force production occurring upon ADP release 12. This motor activity is critical for maintaining structural and functional integrity of cilia in respiratory epithelia and flagella in spermatozoa 12. Clinically, DNAH9 mutations cause primary ciliary dyskinesia (PCD), characterized by impaired mucociliary clearance leading to recurrent respiratory infections, progressive lung dysfunction, and outer dynein arm ultrastructural defects 3. Disease-causing variants result in diminished DNAH9 protein expression and defective flagellar ultrastructure 4. Beyond PCD, DNAH9 mutations associate with male infertility through severe asthenospermia (reduced sperm motility) 54, hypospadias 6, post-infectious bronchiolitis obliterans in children 7, and germline predisposition to myeloid malignancies 89. Dnah9 knockout mice and airway organoid models successfully recapitulate human ciliopathy phenotypes, providing platforms for therapeutic screening 3. Gene-environment interactions with early tobacco smoke exposure modulate bronchial hyperresponsiveness 10.