The NCCR of BKV has been divided into sequence blocks, in order to help visualize rearrangements of the different NCCR variants (Markowitz and Dynan, 1988). The O block contains the origin of replication and TATA box, while the other blocks contain the promoter, the enhancer and the binding sites for transcription factors. In the Dunlop NCCR genomic organization, blocks Q and R, which are usually present in the archetypal organization, are missing, whereas the P block is repeated three times (two of the three P blocks are partially deleted). All the BKV strains used in our study were characterized by a Dunlop NCCR organization. The numbers in brackets indicate the length in nucleotide of each block. a This P block bears a deletion spanning nucleotides 8–25. b This P block bears a deletion spanning nucleotides 65–68.