The function of SPAG1 (sperm associated antigen 1, Ensembl gene identifier ENSG00000104450) is as follows. May play a role in the cytoplasmic assembly of the ciliary dynein arms (By similarity). May play a role in fertilization. Binds GTP and has GTPase activity. {ECO:0000250, ECO:0000269|PubMed:11517287, ECO:0000269|PubMed:1299558}.