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Background: Hepatitis B virus (HBV) has a high mutation rate due to its unusual replication strategy leading to the production of a large number of virions with single and double mutations. The mutations, in turn, are associated with the development of drug resistance to nucleos(t)ide analogs (NUCs) in patients before and during NUCs therapy. Objectives: The current study aimed at investigating the molecular characterization of HBV in Turkish patients with chronic hepatitis B (CHB) infection. Methods: Polymerase chain reaction (PCR) amplificationanddirect sequencing procedures were used to analyze mutations. The detected drug resistance mutations were divided into the nucleos(t)ide analogs primary, partial, and compensatory resistance groups. The amino acid substitutions of hepatitis B surface antigen (HBsAg) were categorized into antiviral drug - associated potential vaccine - escape mutations (ADAPVEMs) and typical HBsAg amino acid substitutions, which included hepatitis B hyperimmunoglobulin (HBIg) - selected escape mutation, vaccine escape mutation, hepatitis B misdiagnosis, and immune - selected amino acid substitutions. Results: The number of patients included in the study was 528 out of which 271 (51.3%) were treatment - naive and 351 (66.3%) were hepatitis B e antigen (HBeAg) - negative. Moreover, 325 (61.6%) were males with a mean age of 38 years (range: 18 - 69). Primary, partial, and compensatory resistance to NUCs was reported in 174 (32.9%) patients. Six different ADAPVEM motifs were determined in both treatment - naive and treatment - experienced patients, namely, sF161L/rtI169X, sE164D/rtV173L, sL172L/rtA181T, sL173F/rtA181V, sS195M/rtM204V, and sS196L/rtM204I. The prevalence of ADAPVEMs and typical HBsAg escape mutations was 5.3% (n = 28) and 34.8% (n = 184), respectively. Conclusions: The analysis of drug resistance should constitute a fundamental part of the follow - up period of patients with CHB undergone treatment with NUCs. The surveillance of development of drug resistance mutations, while receiving treatment for hepatitis B is of paramount importance to monitor and control the emerging resistance. [ABSTRACT FROM AUTHOR]