Identification of the ligand detected by 3E8.1 mAb and antiserum from BL6.9 immunized mice. Western blot analysis of ES cell antiserum identified a unique band (BW lysate, left). The same molecular-weight band was precipitated from both surface-biotinylated BW cell (BW lysate, middle) and BL6.9 cell lysates (right panel) with the 3E8.1 mAb. The band is approximately 520 kDa (BW lysate, right).