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A precore mutant is a variety of hepatitis B virus that does not produce hepatitis B virus e antigen. These mutants are important because infections caused by these viruses are difficult to treat, and can cause infections of prolonged duration and with a higher risk of liver cirrhosis. The mutations are changes in DNA bases from guanine to adenine at base position 1896 , and from cytosine to thymine at position 1858 in the precore region of the viral genome.
The HBV has four genes: S, P, C, and X. The S gene codes for the "major" envelope protein. The largest gene is P. It codes for DNA polymerase. The C gene codes for HBeAg and HBcAg. The C gene has a precore and a core region. If translation is initiated at the precore region, the protein product is HBeAg. If translation begins with the core region, HBcAg is the protein product. HBeAg is a marker of HBV replication and infectivity. The precore region is not necessary for viral replication. Precore mutants can replicate. They are readily detectable by HBV DNA in serum, but hepatitis B e antigen is absent. The X gene codes for HBxAg. The product of the X gene is hepatitis B x antigen. It may be involved in carcinogenesis.
Basal core promoter mutants cause a reduction in HBeAg production. These mutations, A1762T and G1764A, occur in the basal core promoter region of the genome. The frequency of these mutations varies according to the HBV genotype.