Polycystic liver disease (PCLD) is an inherited disorder caused by mutations in either PRKCSH (hepatocystin) or SEC63 (Sec63p). However, expression patterns of the implicated proteins in diseased and normal liver are unknown. We analyzed subcellular and cellular localization of hepatocystin and Sec63p using cell fractionation, immunofluorescence, and immunohistochemical methods. Expression patterns were assessed in fetal liver, PCLD liver, and normal adult liver. We found hepatocystin and Sec63p expression predominantly in the endoplasmic reticulum. In fetal tissue, there was intense expression of hepatocystin in ductal plate, bile ducts, and hepatocytes. However, Sec63p staining was prominent in early hepatocytes only and weak in bile ducts throughout development. In PCLD tissue, hepatocystin was expressed in hepatocytes, bile ducts, and in cyst epithelium of patients negative for PRKCSH mutation. In contrast, the majority of cysts from PRKCSH mutation carriers did not express hepatocystin. Sec63p expression was observed in all cyst epithelia regardless of mutational state. We conclude that hepatocystin is probably required for development of bile ducts and does not interact with Sec63p. The results support the hypothesis that cyst formation in PCLD results from a cellular recessive mechanism involving loss of hepatocystin. Cystogenesis in SEC63 -associated PCLD occurs via a different mechanism.