Required for sperm migration into the oviduct and male fertility by controlling binding of sperm to zona pellucida (PubMed:24501175). May play a role in cell growth (By similarity). Interacts with ADAM3 and TEX101. Strongly expressed in testes and weakly expressed in the epididymis, ovary, and uterus (PubMed:20920470). Expressed in testicular germ cells (TGCs) (PubMed:24501175). Expressed in the testicular seminiferous tubules, in spermatocytes, spermatids, and testicular spermatozoa (PubMed:27005865). Weakly expressed in testes from 18 day postcoitus to 1 day postpartum (dpp), with a plateau starting around 8 dpp; and testicular expression shows two-peak expression at around 14 dpp and 24 dpp, then exhibits stable expression from 6-week after birth onward. Knockout Ly6k mice are viable and show no overt developmental abnormalities. Males are infertile.