Polymerase chain reaction (PCR) select complementary DNA (cDNA) subtraction of hepatitis B x antigen (HBxAg)-positive compared with -negative HepG2 cells resulted in the up-regulated expression of a cellular gene that encodes a transcript of 745 bases and a polypeptide 99 amino acids long. GenBank analysis revealed extensive homology with the amino terminal domain of cellular multidrug resistant proteins (MRP), although overexpression of this gene did not confer an MRP phenotype. In situ ybridization and immunostaining showed colocalized expression with HBxAg in the liver of hepatitis B carriers. Overexpression of this protein stimulated the growth of HepG2 cells in serum-free medium, and partially protected cells from anti-Fas-mediated killing, but did not promote growth in soft agar or tumor formation in nude mice. Introduction of the dominant negative inhibitor of nuclear factor B (I B) into HBxAgpositive HepG2 cells decreased the levels of messenger RNA (mRNA) and protein, uggesting that its up-regulation is nuclear factor B (NF-B) dependent. Hence, HBxAg activation of NF-B may result in the up-regulation of a cellular protein that promotes growth factor-ndependent survival and protects against Fas-mediated killing. This factor may contribute to the persistence of infected hepatocytes during chronic infection, which is important for the later development of epatocellular carcinoma (HCC). (HEPATOLOGY 2001; 34: 146-157.)