The human PSG family has 11 members, and PSG1 is the most abundant family member [29] . We generated recombinant PSG1-FLAG, which consists of the leader peptide, followed by the N, A2, and B2 domains of PSG1 and then a FLAG, V5, and six His tags. The recombinant protein has an approximate molecular mass of 55 kDa and was harvested from the supernatant of stably transfected CHO cells, as described in Materials and Methods. Recombinant PSG1-FLAG protein was sequentially purified on a HisTrap column followed by an anti-FLAG affinity column. These two purification steps rendered a glycoprotein that was more than 90% pure based on Coomassie blue staining (Fig. 1, lane 1) .