Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) of multiciliated respiratory cells and the distal singlet microtubules of monoflagellated spermatozoa (PubMed:36191189). Forms an extensive interaction network cross-linking the lumen of axonemal doublet microtubules (By similarity). . This is the function of Ensembl gene identifier ENSG00000165698 (SPACA9, sperm acrosome associated 9).