Chronic infection of the hepatitis B virus (HBV) is one of the causes leading to liver cancer. The 3.2 kb genome of HBV encodes four proteins: core antigen, surface antigen, a DNA polymerase and the X protein (HBx). The biological functions of HBx are not fully understood. It has been shown that HBx is a potent trans -activator, which activates transcription of many cellular and viral promoters indirectly via protein-protein interactions. These transactivating activities of HBx may contribute to the development of hepatocellular carcinoma. In this paper a truncated mini-HBx(-Cys) ( 18-142) protein, where the cysteines had been either deleted or substituted by serines, was constructed by site-directed mutagenesis and overexpressed as a 6xHis fusion protein in Escherichia coli. The 6xHis-mini-HBx(-Cys) protein was isolated from inclusion bodies, purified by Ni-affinity chromatography under denaturing conditions and refolded by sequential dialysis. The structure of the 6xHis-mini-HBx(-Cys) protein was analyzed by circular dichroism, fluorescence and one-dimensional NMR spectroscopic assays. The data presented here suggest that HBx is unstructured but has a propensity to gain secondary structure under specific experimental conditions. Its conformational flexibility might partially explain its functional complexity, namely its capacity to interact with a wide array of signaling proteins, transcriptional regulators and nucleic acids. © 2005 Elsevier B.V. All rights reserved.126416716574