CD8 (Cluster of differentiation 8) is a transmembrane glycoprotein that belongs to the immunoglobulin (Ig) supergene family. This cell surface protein of 235 amino acids is located on the chromosome 2p11.2. The CD8 chain with about 34 kDa is made of an immunoglobulin-like beta-sandwich folding and 114 amino acid residues. CD8 is found on most cytotoxic T lymphocytes and mediates efficient cell-cell interactions upon the immune system. In T-cells, it serves primarily as a co-receptor for MHC class I molecule: peptide complex and specific for this protein. To function, CD8 will form a dimer composed of a pair of CD8 chains. The most common combination consists of two alpha (CD8-α) chains as well as an alpha chain and a beta (CD8-β) chain. PSCA (Prostate stem cell antigen) is a cell surface protein of Thy-1/Ly-6 family anchored by a glycosylphosphatidylinositol (GPI) linkage. It is located on chromosome 8q24.2 and encodes a 123-aa glycoprotein with an N-terminal signal sequence, a C-terminal GPI-anchoring sequence, and multiple N-glycosylation sites. PSCA shares 30% homology with a surface marker of immature lymphocytes, called stem cell antigen type 2 (SCA-2). Just as SCA-2, PSCA is attached to the cell membrane that can be separated by a GPI-specific phospholipase. In humans, PSCA is highly expressed in the basal and secretory epithelial cells of the normal prostate and also detected in placenta, bladder, stomach, colon, and weakly in the kidney. It is up-regulated in some cancers and is found in more than 80% of prostate carcinomas.