Probable C-mannosyltransferase that mediates C-mannosylation of tryptophan residues on target proteins. Required during spermatogenesis for sperm head elongation and acrosome formation. Also plays a role in acrosome attachment to the nuclear envelope. Interacts with FAM209. Restricted to the inner nuclear membrane facing the acrosomal vesicle. The N- and C-termini are oriented towards the nucleoplasm (PubMed:22764053). Colocalizes with FAM209 at the inner nuclear membrane (PubMed:34471926). Predominantly expressed in testis. Present in testis but absent from epididymal sperm (at protein level). Deficient male exhibit sterility associated with globozoospermia. Belongs to the dpy-19 family.