Construction of the pBB vector. We first constructed a pBBSmaI vector by inserting a linker of 59-SalI-SacII-KpnI-SmaI-ApaI-SphI-KpnI-HindIII-XhoI-39 into a vector backbone (pProEX HTb vector; Invitrogen) generated from the B19V infectious clone pM20 [78] by removing all of the B19V sequence (SalI-digestion). All cloning work was carried out in the Escherichia coli strain of Sure 2 (Agilent, La Jolla, CA). All the nucleotide numbers of HBoV1 refer to the HBoV1 full-length genome (GenBank accession no.:JQ923422).