Adult tree shrews (Tupaia belangeri chinensis) were housed in a Tupaia animal facility at the National Institute of Biological Science, Beijing. All studies were performed in accordance with institutionally approved protocols and adherent to guidelines of the National Institute of Biological Sciences Guide for the care and use of laboratory animals. PTH cells were obtained from anesthetized Tupaia (100-150 g) with a two-step perfusion method as previously described (Walter et al., 1996) . Cell suspensions after perfusion were filtered through a 70-μm cell strainer and centrifuged at 50 g for 3 min. The cell pellet containing PTHs was resuspended in plating medium of Williams E medium supplemented with 10% FBS, 5 μg/ml transferrin, 5 ng/ml sodium selenite, 2 mM L-glutamine, 100 U/ml penicillin, and 100 μg/ml streptomycin. The cells were then plated on collagen-coated cell culture dishes or plates. 4 hr after plating, medium were changed to primary hepatocytes maintenance medium (PMM), that is, Williams E medium supplemented with 5 μg/ml transferrin, 10 ng/ml EGF, 3 μg/ml insulin, 2 mM L-glutamine, 18 μg/ml hydrocortisone, 40 ng/ml dexamethasone, 5 ng/ml sodium selenite, 2% DMSO, 100 U/ml penicillin, and 100 μg/ml streptomycin. Cells were maintained in 5% CO 2 humidified incubator at 37°C with regular medium change every 2-3 days.