Using hepatopioetin (HPO) as “bait” in an yeast two-hybrid screen resulted in the identification of JAB1(a protein initially as a co-activator of c-Jun) as a putative HPO binding partner.Then the full length cDNA of JAB1 was obtained from human fetal liver cDNA library by PCR for GST-JAB1 expression.The binding assay showed that JAB1 binded to rHPO and HPO expressed in COS7 cells in vitro.