AIM: To verify whether “defective” mutations existed in hepatitis D virus (HDV). METHODS: Hepatitis delta antigen (HDAg)-coding sequences were amplified using Pfu DNA polymerases with proof-reading activities from sera of five patients with chronic hepatitis D. Multiple colonies were sequenced for each patient. Pfu analyzed a total of 270 HDV clones. Three representative defective HDV clones were constructed in expression plasmids and transfected into a human hepatoma cell line. Cellular proteins were extracted and analyzed by Western blot. RESULTS: Four of five cases (80%) showed defective HDV genomes in their sera. The percentage of defective genomes was 3.7% (10/270). The majority (90%) of the defective mutations were insertions or deletions that resulted in frameshift and abnormal stop translation of the HDAg. The predicted mutated HDAg ranged from 45 amino acids to >214 amino acids in length. Various domains of HDAg associated with viral replication or packaging were affected in different HDV isolates. Western blot analysis showed defected HDAg in predicted positions. CONCLUSION: “Defective” viruses do exist in chronic HDV infected patients, but represented as minor strains. The clinical significance of the “defected” HDV needs further study to evaluate. AIM: To verify whether “defective ” mutations existed in hepatitis D virus (HDV). METHODS: Hepatitis delta antigen (HDAg) -coding sequences were amplified using Pfu DNA polymerases with proof-reading activities from sera of five patients with chronic hepatitis D. Multiple colonies were sequenced for each patient. Pfu analyzed a total of 270 HDV clones. Three representative defective HDV clones were constructed in expression plasmids and transfected into a human hepatoma cell line. Cellular proteins were extracted and analyzed by Western blot. RESULTS: The majority of (90%) of the defective mutations were insertions or deletions that resulted in frameshift and abnormal (80%) had defective HDV genomes in their sera. The percentage of defective genomes was 3.7% (10/270). stop translation of the HDAg. The predicted mutated HDAg ranged from 45 amino acids to> 214 amino acids in length. Various domains of HDAg associated with viral replication or packaging were affected in different HDV isolates. Western blot analysis showed defected HDAg in predicted positions. CONCLUSION: “Defective ” viruses do exist in chronic HDV infected patients, but represented as minor strains. The clinical significance of the “defected ” HDV needs further study to evaluate.