His-tagged TPOs in the culture supernatants and cell lysates of COS-1 cells were adsorbed on a Ni-NTA resin column. The eluates were precipitated with the Ni-NTA resin, and the precipitates proteins were separated by SDS-PAGE and probed with anti-His(C-term) antibody. Lanes 1 and 5, TPO-1; lanes 2 and 6, TPO-5; lanes 3 and 7, TPO-6; lane 4, untransfected COS-1 cells. In lane 5, the band corresponding to TPO-1 appears to be masked by nonspecific bands.