Here, based on the recent finding of HBx (X-gene product of hepatitis B virus) as the inducer of Jak1, we investigated the mechanism for the HBx-mediated host cell regulation and found that (i) HBx associates specifically with Jak1 in vivo, (ii) HBx itself forms a dimer which leads to juxtaposition of associated Jak1 and subsequent activation of the tyrosine kinase activity of Jakl; (iii) HBx-mediated activation of the promoters containing AP-1-, NF-kappaB-, SRE-, and SIE-sites is dependent on the activation of Jak1; (iv) Jak1, once activated by HBx, induces Ras activity through recruitment of Grb2 and induces tyrosine phosphorylation of Raft, but not shc. These findings show that previously reported functions of HBx, such as activation of multiple signaling pathways and transcriptional activation are attributable to HBx-mediated Jak1 activation.