A number of truncated E proteins have been successfully produced by co-expression with prM (e.g., references [30, 39] ), while the pr-E structural studies were based on a secreted hybrid protein containing truncated prM linked to truncated E [34] . Previous studies indicated that full-length TBEV prM could fold correctly when expressed in the absence of E protein [37] , suggesting that production of pr peptide alone might be possible. We generated a construct based on residues 1-86 of DENV2 prM, truncating pr just before the start of the furin cleavage recognition site at residue 87 (Fig. 1A) . This sequence was linked to a mammalian signal peptide at the N-terminus and to an affinity tag at the C-terminus, and expressed in 293T cells. The protein was isolated in a highly purified form by affinity chromatography and gel filtration (Fig. 1B) , and was recognized by mAb prM-6.1 against prM [40] (data not shown). The pr peptide migrated at a position of ,17 kDa in reducing SDS-PAGE, in keeping with its predicted size of 13 kDa plus the presence of carbohydrate due to the glycosylation site at position 69. This carbohydrate was removed by Peptide N-glycosidase F (PNGase F) to give a peptide of the predicted size. The protein was largely resistant to Endoglyosidase H (Endo H) digestion, indicating maturation of the carbohydrate chain as the protein transited through the Golgi complex. A mobility shift was observed upon reduction of pr, in keeping with the presence of 3 disulfide bonds in the structure of pr [34] .