RSPH4A (radial spoke head component 4A) is an essential structural protein component of the axonemal radial spoke head that plays a critical role in ciliary motility 1. The protein is required for the assembly of all three radial spoke heads (RS1, RS2, and RS3) in motile cilia, with cryo-electron tomography revealing that RSPH4A-deficient cilia lack all triplet spoke heads 2. Mechanistically, RSPH4A forms part of a reconstituted murine radial spoke head core complex alongside Rsph1, Rsph3b, and Rsph9, creating a brake pad-shaped structure that contacts central pair projections to coordinate ciliary motility 3. In RSPH4A-deficient cilia, the radial spoke heads of RS1 and RS2 are missing, along with defects in adjacent arch domains, resulting in impaired planar beating patterns 4. Disease-wise, mutations in RSPH4A cause primary ciliary dyskinesia (PCD), characterized by chr6 respiratory infections, bronchiectasis, and infertility, but notably without laterality defects 5. A founder mutation (c.921+3_921+6delAAGT) is prevalent in Puerto Rican populations, causing decreased ciliary beat frequency and rotational beating patterns 6. Clinically, RSPH4A-related PCD can be diagnosed through high-speed video microscopy analysis, transmission electron microscopy, and immunofluorescence techniques 5.