['This study was conducted to clarify the resistance profile of a novel mutation pattern emerging during lamivudine (3TC) therapy and showing cross-resistance to adefovir dipivoxil (ADV) in a patient with chronic hepatitis B.', 'Successful suppression of hepatitis B virus (HBV) replication by sequential therapy of 9 MU thrice weekly interferon (IFN) and 3TC was followed by genotypical resistance detected at month 28 of therapy (month 19 of lamivudine treatment). ADV was added to 3TC therapy on month 44 of antiviral treatment. Neither alanine aminotransferase normalization nor a stable decrease in HBV viral load was observed, although ADV was used for more than 40 months. The HBV pol region was amplified from serum samples obtained before and after ADV treatment. The complete genome was cloned into a TA vector. PCR products and 7-10 clones from each cloned vector were sequenced. A novel mutation, A181S, in the reverse transcriptase gene leading to a conversion of W172C in the overlapping surface antigen gene was detected along with a M2041 mutation. The complete genome comprising the A181S+M2041 pattern was cloned into an expression vector and its in vitro susceptibility to 3TC, ADV, tenofovir (PMPA), clevudine (L-FMAU) and emtricitabine (FTC) were determined in transiently transfected Huh7 cells. This mutation pattern displayed more than 1000-fold resistance to the nucleoside analogues 3TC and FTC and approximately sixfold resistance to L-FMAU, while it confers 28.23- and 5.57-fold resistance for the nucleotide analogues ADV and PMPA, respectively.'] Does a novel mutation pattern emerging during lamivudine treatment show cross-resistance to adefovir dipivoxil treatment?