The lymphocyte antigen 6 complex locus K (LY6K), a member of cancer-testis antigen was initially identified as a molecular marker for head-and-neck squamous cell carcinoma as well a breast cancer. LY6K is a GPI-anchored membrane protein that is specifically associated with germ cell marker TEX101 and is strongly observed in testis, but only weakly in other tissues. LY6K mRNA was found to be upregulated in numerous bladder cancers due to gene amplification. Furthermore, knockdown experiments using LY6K siRNA reduced cell growth, migration and invasion in bladder carcinoma cell lines, suggesting that LY6K contributes to bladder cancer development.