Essential for sperm flagellar assembly and male fertility. Interacts with RSPH1, RSPH4A, RSPH9 and RSPH10B. Expressed in testis (at protein level). First detected during the haploid phase of spermatogenesis when secondary spermatocytes begin to appear at about postnatal day 18 (PubMed:30185526). Expression is maintained in mature sperm (PubMed:30239614). Phosphorylated by PKA. Phosphorylation increases in capacitated sperm. Male mutant mice are infertile. Mutant spermatozoa exhibit shorter tails, misshapen heads and immotility. Manchette removal is impaired. Belongs to the flagellar radial spoke RSP4/6 family.