Semenogelin (Sg) is the major protein involved in gelatinous entrapment of ejaculated spermatozoa, which plays an important role on the regulation of sperm motility and fertilization. We investigated deletion in Sg-I and -II genomic genes with the objective of diagnosis of asthenozoospermia. As a result, heterozygous deletion of a repeat site in exon 2 of the Sg-I genomic gene was identified. The repeat site-deleted Sg-I protein was also detected together with full-sized Sg-I in semen ejaculated from infertile patients who were diagnosed as having the heterozygous deletion of a repeat site in exon 2 of the Sg-I genomic gene. However, the deletion was found not only in male infertile patients but also in fertile men. The deletion frequency was 1.1% in infertile patients and 2.8% in fertile men. The deletion was found to have no effect on testicular volume, endocrine hormone concentration and sperm quality. In addition, the wild-type, the repeat site-deleted and active site-deleted Sg-I proteins were produced in recombinant baculovirus-infected Sf21 cells and their seminal plasma motility inhibitor (SPMI) activities were determined. The repeat site-deleted Sg-I had similar SPMI activity to the full-sized Sg-I, while the active site-deleted Sg-I had almost lost SPMI activity. In conclusion, the deletion of a repeat site in the Sg-I genomic gene found in infertile patients and fertile men does not affect sperm quality and male infertility.

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