Microtubule inner protein (MIP) part of the dynein-decorated doublet microtubules (DMTs) in cilia axoneme, which is required for motile cilia beating (PubMed:36191189). May play a role in the control of meiotic division and germ cell differentiation through regulation of pairing and recombination during meiosis. Required for sperm flagella assembly (By similarity). May play a role in the assembly and function of the outer dynein arm-docking complex (ODA-DC). ODA-DC mediates outer dynein arms (ODA) binding onto the axonemal doublet microtubules (PubMed:30148830). {ECO:0000250|UniProtKB:Q61884, ECO:0000269|PubMed:30148830, ECO:0000269|PubMed:36191189}. This is the function of Ensembl gene identifier ENSG00000138587 (MNS1, meiosis specific nuclear structural 1).