DNAH9 encodes a dynein axonemal heavy chain that is a core component of outer dynein arms (ODAs), which are essential motor protein complexes required for ciliary and flagellar beating. DNAH9 generates force toward the minus ends of microtubules through ATPase activity, with the power stroke occurring upon ADP release. This heavy chain localizes to the distal axonemal compartment and is critical for the structural and functional integrity of motile cilia in respiratory epithelium and sperm flagella. Recessive DNAH9 loss-of-function mutations cause primary ciliary dyskinesia (PCD) with situs inversus totalis or heterotaxy, alongside respiratory dysfunction 1. Mutant cilia exhibit distally impaired bending and lack other ODA components including DNAH5 and DNAI proteins 1. DNAH9 variants have been identified in infertile men with severe asthenospermia, resulting in outer dynein arm deficiency in sperm axonemes 2, and in pediatric patients with post-infectious bronchiolitis/bronchitis obliterans following Mycoplasma pneumoniae or adenoviral infection 3. Pathogenic DNAH9 mutations also associate with congenital heart disease 4. At the population level, gnomAD v4.1 classifies this gene as loss-of-function tolerant (LOEUF=0.78); this is distinct from clinical pathogenicity in disease contexts where biallelic mutations cause significant morbidity. Mouse models and airway organoids recapitulate PCD phenotypes and enable drug screening for ciliary dysfunction 5.