CsMRP4 was mainly localized in the mesenchymal tissues in our study. In trematodes, several transporters have been found to be expressed in mesenchymal tissues. BSEP and MRP1 of adult Fasciola hepatica have been shown to be localized in not only mesenchymal tissues but also the tegumental cell layer, implying that the two transporters facilitate the diffusion of bile salts and chemicals in flukes . CsMRP7, which might be involved in drug resistance, has also been found to be expressed in the mesenchymal tissues . It is therefore speculated that mesenchymal tissues may be regions of strategic importance for transporter functions throughout the body of flukes.