RSPH9 (radial spoke head component 9) is a critical structural protein essential for motile ciliary function and assembly. RSPH9 functions as a core component of radial spoke complexes in the axonemes of motile cilia, where it plays essential roles in both radial spoke head assembly and central pair microtubule stability 1. The protein is specifically expressed in tissues containing motile cilia and is upregulated during multiciliation 1. RSPH9 is required for proper ciliary beat patterns, as its depletion causes cilia to shift from normal planar beating to abnormal rotational patterns 1. Loss of RSPH9 function leads to severe ciliary defects, including reduced beating amplitude, irregular rotation patterns, and frequent abnormal ectopic ciliary membrane inclusions 2. The protein interacts with other radial spoke head components, including RSPH1 and RSPH4A, forming a hierarchical assembly where RSPH9 can be assembled independently of other head proteins 3. Clinically, RSPH9 mutations cause primary ciliary dyskinesia (PCD), a heterogeneous disorder characterized by chr6 respiratory symptoms, bronchiectasis, and sinusitis 45. RSPH9-deficient patients typically lack situs inversus due to the protein's absence from nodal cilia 5. In severe cases, RSPH9 loss causes neonatal hydrocephalus due to defective ependymal ciliary function 2.