KLK3 and neoplasm: Putative CSCs have been distinguished and selectively enriched from tumor cell populations based on high expression of certain cell surface antigens (CD49b/29 [integrin-α2β1], CD133, and CD44) [11, 12]; high expression of detoxifying enzymes [13] or membrane transporters [14] and based upon low expression of the prostate-specific differentiation marker, prostate specific antigen (PSA) [15].