This dreadful Hepatitis B Virus (HBV) is small in size. The genome of this virus is a partial double stranded circle. When made fully double stranded, this genome carries about 3000 base pairs, compared to 200 kilo base pairs of the genome of the smallpox virus. These 3000 base pairs encode an envelope protein, a core protein, a polymerase essential for virus replication and a very special X protein, named so because its function was unknown when it was named.  A new article in the World Journal of Gastroenterology by Dr. Kremsdorf and her associates addresses this question.  They have been trying to elucidate some of the functions of this X protein involved in liver pathogenesis during HBV infection. They first established a system in which the gene for X protein is permanently incorporated into mouse genome. With transgenic mice expressing X protein, they could research many different impacts of the protein on the host.  Their first exciting discovery was the inhibition of liver cell proliferation by X protein. This discovery raised a novel mechanism on how HBV causes liver diseases. Recently, the team further investigated how X protein inhibited the liver cell proliferation. These researchers studied the expression of 5376 genes in the transgenic mice using DNA microarray technique, which allowed simultaneous analysis of all 5376 genes. Their results indicated a decreased activity of those genes required for gene transcription and cholesterol metabolic pathway. This not only confirmed the previous observation, but showed how the molecular mechanism of how Hepatitis B virus X protein inhibits the liver regeneration.  These new discoveries should improve our knowledge of the implication of the viral proteins in the pathogenesis of HBV infection. This should allow participation in the design of new and more effective treatments for HBV patients.