The cDNA encoding the PSG1 leader peptide (L), N, A2, and B2 domains followed by a FLAG tag (Sigma) without the stop codon was synthesized by GenScript Corporation. The cDNA was subcloned into the BglII and NotI sites of the pEF1/V5-His vector (Invitrogen), resulting in the in-frame addition of the nucleotide coding for the V5 and histidine (His) tags at the C-terminus. The final construct, designated here as PSG1-FLAG, codes for a secreted PSG1 protein composed of the N, A2, and B2 domains followed by three tags-FLAG, V5, and His-at the C-terminus. To generate PSG1-Fc, the PSG1 cDNA encoding the L, N, A2, and B2 domains of PSG1 was subcloned inframe into the EcoRI-BglII sites of pFuse-IgG1 e3-Fc1 (InvivoGen), resulting in the addition of the hinge region, CH2 and CH3 domains, of the IgG heavy chain at the C-terminus.