C2 and congenital hydrocephalus: Biochemical analysis revealed that HYDIN interacts with CPC1 (mouse SPEF2) and KLP1 (mouse KIF9),41 and the absence of HYDIN results in a complete loss of detectable KIF9.39 RNA interference targeting HYDIN in Chlamydomonas results in short flagella lacking the C2b projection,41 and the deficiency of HYDIN is linked to congenital hydrocephalus and fetal death in mice,42 both of which align with the involvement of HYDIN in the assembly of C1b, the bridge, C2b, and C2-MOSP (Fig. 3).