Involved in fertilization by facilitating sperm penetration of the zona pellucida (PubMed:19596312, PubMed:22817830). May promote spermatocyte apoptosis, thereby limiting sperm production. In adults, may reduce testosterone synthesis in Leydig cells (PubMed:18818293). Is not essential either for development or fertility (PubMed:19596310). Interacts with SLXL1; Co-localize in seminiferous tubules (PubMed:21698294). Interacts with SLY (PubMed:19176879). Localized specifically to the crescent shaped acrosome at the apex of the sperm head. Testis-specific. Abundant in the seminiferous tubules where it is associated with developing spermatocytes. Expressed only in testis (at protein level) (PubMed:15892050, PubMed:19596310). Not detectable on postnatal days 4 and 9 but after day 18 it gradually increased as the development of testes progressed. Expressed at high levels in testis and at weak levels in epididymis (PubMed:22817830). Expressed in the embryo only after day 15. In the adult, expressed only in developing spermatocytes. Expressed in the developing cochlea. Detected only in developing spermatocytes and spermatids in seminiferous tubules. Moreover, first appears specifically in zygotene/pachytene spermatocytes. Found in puddles in the pachytene spermatocytes of all stage tubules and then in crescent shaped structures characteristic of acrosomes in early step spermatids. Detected in mature sperm (PubMed:15892050). Expressed strongly in trophoblast stem cells and further up-regulated in trophoblast giant cells. Expression is maintained in post-implantation placental tissues in utero. Highly expressed from 7.5 dpc trophectoderm to 12.5 dpc placenta. Expression remains baseline in postimplantation embryonic tissues (PubMed:19596312). Contains a N-terminal domain similar to that of the N-terminal section of DKK3. N-glycosylated during spermatogenesis. Not N-glycosylated in mature sperm. Nullizygous embryos Dkkl1 mice develop into viable, fertile adults.