Cathepsins (CTSs), a family of lysosomal cysteine proteases, and their inhibitors (CSTs) play a critical role in remodeling of the uterine endometrium and placenta for the establishment and maintenance of pregnancy in many animal species including rodents, sheep, cow and pigs. It has been shown that the high rate of pregnancy failure by somatic cell nuclear transfer (SCNT) is associated with abnormal placental development. Our previous study has shown that CST6 is highly expressed in the uterine endometrium from mid to late pregnancy in pigs. In this study, to understand whether appropriate endometrial and placental tissue remodeling occurs in the uterine endometrium from gilts with conceptuses derived from SCNT during pregnancy in pigs, we investigated expression of CST6 in the uterine endometrium. Uterine endometrial tissues were obtained from gilts that carried SCNT-derived normal conceptuses (NT-No) and abnormal conceptuses (NT-Ab), and from gilts carrying conceptuses from natural mating (Non-NT) on D114 of pregnancy. Immunoblot analysis showed that CST6 protein levels in the endometrial tissues of gilts carrying NT-No were lower than those of gilts carrying Non-NT. The levels of CST6 protein in the endometrial tissues of gilts carrying NT-Ab decreased even more than those of gilts carrying NT-No. These results indicate that decreased expression of CST6 in the endometrium with NT-No and NT-Ab reflects inappropriate endometrial tissue remodeling and pregnancy failure of pigs with SCNT derived conceptuses and that CST6 plays an important role for the maintenance of pregnancy in pigs. * This work was supported by the Next Generation BioGreen 21 program (#PJ007997), RDA, Republic of Korea.