It is notable that the HBoV1 terminal hairpins appear to be hybrid relicts of the prototype bocavirus BPV1 at the LEH, but of MVC at the REH [28] . Replication of HBoV1 DNA in HEK293 cells revealed typical replicative intermediates of parvoviral DNA. Although the head-tail junctions are unexpected in the replication of autonomous parvoviruses, they were likely generated during the cycle of rolling hairpin-dependent DNA replication [46] . Therefore, we believe that the replication of HBoV1 DNA basically follows the model of rolling hairpin-dependent DNA replication of autonomous parvoviruses, with terminal and junction resolutions at the REH and LEH, respectively [46] . The replication of parvoviral DNA depends on entry into S phase of the cell cycle or the presence of helper viruses [46, 47] . In this regard, it is puzzling that mature, uninjured airway epithelia are mitotically quiescent (,1% of cells dividing) [48] [49] [50] , as are the majority of the cells in polarized HAE (in the G0 phase of the cell cycle). However, recombinant adeno-associated virus (AAV; in genus Dependovirus of the family of Parvoviridae) infects HAE apically and expresses reporter genes [51] [52] [53] . Gene expression by recombinant AAV requires a conversion of the ssDNA viral genome to a doublestranded DNA form that is capable to be transcribed [54] . This conversion involves DNA synthesis. Hence, we hypothesize that HBoV1 employs a similar approach to synthesize its replicative form DNA. Notably, wild type AAV infected primary HAE apically and replicated when adenovirus was co-infected [55] . The exact mechanism of how HBoV1 replicates in normal HAE will be an interesting topic for further investigation.