PCR experiments showed that an amplicon specific to CsTegu20.6 was detected in the adult worm and metacercariae of C. sinensis, but not in the egg (Figure 9A). A full-length cDNA clone of CsTegu20.6 was used as the positive control. We also confirmed that native CsTegu20.6 was expressed in adult worm and metacercariae using anti-rCsTegu20.6 sera (Figure 9B). As previously described in Figure 5C, CsTegu20.6 was predicted to form homodimers, which can be an unidentified protein complex of molecular weight between the 37-kDa and 50-kDa bands. The lack of expression in the egg possibly arises from the fact that tegumental proteins are differentially expressed in the body according to developmental stages . By the manipulation of its biological aspects, the tegument serves as an important biological means of protection. When encysted metacercariae migrate to the common and hepatic bile ducts in the mammalian host and then grow to adult worms, the juvenile worms are completely exposed to the host immune attack and toxic bile acids during the processes of migration and development . Conversely, similar expression between the adult worm and metacercariae suggests that CsTegu20.6 might be useful as a potential antigen for the early diagnosis of clonorchiasis, which would have the advantage that C. sinensis infections could be detected prior to egg deposition .