An internal domain of the hepatitis B virus X antigen is necessary for transactivating activity.

A number of deletion mutants of the hepatitis B viral X antigen had been constructed and assayed for their ability to transactivate heterologous viral regulatory elements. Neither the N-terminal nor the C-terminal amino acid residues were required for transactivating activity. Transactivating activity that resided within amino acid residues 32 to 148 of the X antigen did not show any target DNA sequence or cell line specificity.