RSPH1 (radial spoke head component 1) is a structural protein essential for ciliary and sperm motility, functioning as a core component of axonemal radial spoke head complexes 1. The radial spoke head consists of RSPH1 alongside RSPH3b, RSPH4a, and RSPH9, forming a brake pad-shaped structure that contacts the central pair apparatus to coordinate ciliary beating 2. RSPH1 mutations cause primary ciliary dyskinesia (PCD), an inherited respiratory disease characterized by defective mucociliary clearance and infertility 1. Loss-of-function RSPH1 mutations result in central pair agenesis with distinctive central pair microtubule instability and loss of the entire radial spoke head structure, including degradation of RSPH4A and RSPH9 13. Cryo-electron tomography reveals that RSPH1-mutant cilia display previously unknown primary structural defects and heterogeneous secondary defects 3. Clinically, RSPH1-PCD patients typically present with normal body laterality and less severe lung disease compared to other PCD variants, possibly due to residual ciliary motility 4. Immunofluorescence analysis effectively diagnoses RSPH1 defects, which can result from loss-of-function mutations or missense variants 5. Beyond ciliary dysfunction, RSPH1 has been identified as a regulator of GSTA1 expression in cancer cell drug tolerance contexts 6, and variants have been associated with disorders of sex development 7.