SPACA9 is a microtubule inner protein (MIP) that localizes to the axoneme of ciliated and flagellated cells. As a polyspecific MIP, SPACA9 binds to both doublet microtubules of multiciliated respiratory cells and distal singlet microtubules of human spermatozoa 1. Within doublet microtubules, SPACA9 forms intralumenal striations in the B tubule, while in sperm singlet microtubules it adopts noncontinuous spiral conformations, suggesting structural flexibility in response to different microtubule architectures 1. The protein functions as part of an extensive cross-linking network within the axonemal microtubule lumen, contributing to the structural integrity and organization of ciliary and flagellar axonemes. SPACA9's presence in both respiratory and reproductive systems indicates its fundamental role in axoneme assembly and function. The identification of SPACA9 as a conserved lumenal protein component provides critical structural references for understanding ciliopathy-causing mutations and enables cryo-EM analysis of patient-derived ciliary samples 1. While direct disease associations have not been characterized in the provided abstracts, SPACA9's essential role in microtubule organization suggests that mutations could compromise ciliary and sperm flagellar function, potentially contributing to ciliopathies and male infertility.