Component of the axonemal radial spoke head which plays an important role in ciliary motility (PubMed:32203505). Essential for triplet radial spokes (RS1, RS2 and RS3) head assembly in the motile cilia (PubMed:32203505). Interacts with RSPH6A. Radial spoke. Expressed in the trachea, ependymal cells and oviduct (at protein level) (PubMed:32203505, PubMed:30239614). Expressed in brain, kidney and testis. Mice show hydrocephalus and the tracheal and ependymal cell cilia show clockwise rotation motion instead of planar beating (PubMed:32203505). All three types of radial spoke heads (RS1, RS2 and RS3) are missing in the tracheal cilia (PubMed:32203505). Belongs to the flagellar radial spoke RSP4/6 family.