Transactivation by expression of the hepatitis B virus X protein with an inducible system

We describe here the conditional expression of the hepatitis B virus X prot ein using the inducible system controlled by a tet-responsive promoter. Ind uction 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 cel l 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.