Liver damage caused by chronic hepatitis B virus (HBV) infection may be enhanced through the selection of deleted HBV preS mutants by intracellular accumulation of viral proteins and subsequent cell death. However, the prevalence and impact of such mutants on the clinical course of infection have not yet been studied in children. Serum samples from 60 children (mean age 9.8 y) were investigated by means of PCR and direct sequencing of the entire preS region. Only one patient (1.5%) was found with a mixed HBV population of a deletion spanning 183 nucleotides and wild-type sequences. This mutation alters the HBV largesurface protein and removes the small-surface promoter. To clarify the significance of this mutation, we studied 14 serial serum samples of the child within a follow-up of 10 y. After occurrence of the mutation, the liver enzymes increased, despite seroconversion to anti-HBe. Transfection of an HBV expression construct containing this deletion into human hepatoma cells by using an HBV in vitro replication system showed that the mutant lost the ability of nucleocapsid packaging as a result of alteration of the transmembrane topology of the large surface protein. This effect could not be restored by coexpression of wild-type largeor small-surface proteins in trans. In conclusion, the circulation of HBV preS deletion mutants is rare in childhood. However, our functional and clinical follow-up studies in one child suggest that such a mutant may have the potential to aggravate liver inflammation, especially if corroborated with larger numbers of children. However, in most of these studies, the patients were under exceptional courses of HBV infection, such as immunosuppression or fulminant liver failure, and therefore only a limited number of subjects have been analyzed. Thus, more evidence that HBV variants can also become clinically relevant for chronic carriers with common courses of infection is still required.Mutations in the HBV envelope preS region have been shown to be associated with ongoing liver disease both on the clinical and the functional level (9 -11). This is due to the pivotal function of surface peptides in virus morphogenesis that envelop the DNA-containing capsid with three viral proteins, termed the large (L), middle (M), and small (S) envelope proteins. These polypeptides are encoded in a single open reading frame of the HBV genome that contains three start codons and a common stop codon. Accordingly, the S region is translated by all three proteins, whereas M and L are synthesized with amino-terminal extensions encoding the preS2 region and the preS2 plus preS1 region (preS), respectively (see Fig. 1). Among the three envelope proteins, particularly the L protein has a crucial role in the viral life cycle. It has the unusual ability to adopt two transmembrane topologies in the endoplasmic reticulum (ER) membrane (see Fig. 2 (9). The cytosolic orientation of L interacts with the nucleocapsid while the luminal orientation is required to localize the preS region on the surface of secret...