HOOK1 is required for spermatid differentiation. It is probably involved in the positioning of the microtubules of the manchette and the flagellum in relation to the membrane skeleton (By similarity). It is a component of the FTS/Hook/FHIP complex (FHF complex). The FHF complex may function to promote vesicle trafficking and/or fusion via the homotypic vesicular protein sorting complex (the HOPS complex) [taken from NCBI Entrez Gene (Gene ID: 51361)]. [taken from the Universal Protein Resource (UniProt) www.uniprot.org/uniprot/Q9UJC3].