The region encoding the stem-loop structure of the HBV cis-encapsidation signal was sequenced in 87 patients with Various terms of hepatitis. Altogether, 20 nucleotide substitutions were found; 15/20 were predicted to increase the stability of the stem-loop structure. Substitutions likely to decrease the stability of the cis-encapsidation signal were not observed. (C) 1994 Academic Press, Inc.