To address the underlying causes of the dramatically decreased in vivo fertilization rate of Prss55−/− males and thus male infertility, we evaluated the in vivo sperm migration in the female genital tract by counting sperm present in the oviduct 3.5 h after coitus and by tracing the migration of EGFP-expressing sperm from uterus into oviduct. Here, PRSS55 is linked to male infertility.