FhGST-S1 was first identified in adult liver fluke in S-hexyl-GSH affinity isolated fractions of cytosol [11] . Western blots confirmed the presence of FhGST-S1 in NEJs and adult flukes and further enabled us to identify the Sigma GST in relative abundance in egg extracts, suggesting that it may play a metabolic role in embryogenesis/reproduction (Figure 3) . Western blot analyses demonstrate that FhGST-S1 is consistently expressed during the course of in vitro parasite embryonation (days 1-9, only data for days 2, 7 and 9 shown in Figure 3 ). In contrast, immunoblot analysis of freshly voided (day 0) eggs reveals that expression of the Sigma class GST is greatly reduced at the time of voiding from the host (Figure 3) . However, immunolocalisation studies of adult parasites revealed an abundance of FhGST-S1 in the vitelline cells and eggs, emphasising the likely importance of this enzyme in egg formation and development. Some staining was also found in the parasite parenchyma and tegument, also suggesting a role at the hostparasite interface ( Figure 4) . Indeed, FhGST-S1 was detected in ES products of adult fluke cultured in vitro ( Figure 3 ) suggesting that the protein could, in principle, come into contact with the host immune system as it is released from the tegument during tegumental turnover and sloughing of the fluke body surface. Figure 2 . Detection of prostaglandin synthase activity of rFhGST-S1 via a mass spectrometry approach. A coupled assay with rFhGST-S1 and COX-1 catalyses the conversion of arachidonic acid to the H2 form before the prostaglandin isomer is converted to either the D or E form. Nano-LC/MS analysis allowed detection of both PGD2 (A) and PGE2 (B) in the assay mixture with the PGD2 form being the more abundant of the two prostanoids (C). Boxed figures above peaks show the fragmentation ions specific to detection of PGD2 (a) and PGE2 (b) according to the method of Schmidt et al. [39] . doi:10.1371/journal.pntd.0001666.g002