We describe here the conditional expression of the hepatitis B virus X protein using the inducible system controlled by a tet-responsive promoter. Induction of the X protein in Rat-2 fibroblasts activated transcription from a heterologous gene promoter and stimulated the DNA-binding activity of NFkB. The ability to produce this biologically active X protein in a stable cell line will accelerate the elucidation of the function and mechanisms of X and eventually help us understand the role of X in natural viral infection and carcinogenesis.
