The TRA-1-81 antibody reacts with a 200-240 kD cell surface antigen expressed by human embryonic stem cells (ESC), embryonic germ cells (EG), and embryonal carcinoma cells. The expression of TRA-1-81 on human ESC is downregulated upon differentiation. Further studies have shown that TRA-1-81 is a neuroaminidase-resistant carbohydrate epitope expressed on podocalyxin, a member of mucin-like glycoprotein with CD34 family. Podocalyxin is a type I tranmembrane sialoprotein with anti-adhesion property.