Northwestern Blot has been used for isolation of cDNAs encoding proteins interacting with HBsAg posttranscriptional regulatory element (PRE). One expression cDNA clone which showed specially binding to PRE was isolated from λgt11 cDNA expression library prepared with human hepatoma cell line HepG2. The size of this PRE-interacting protein(PIP) clone was identified to be about 1 kb by PCR and digestion with restriction endonuclease EcoRⅠ.The results showed that Northwestern blot with some modifications is a useful method for isolation of expression clones of genes encoding RNA-binding proteins.