Semiquantitative assessment of retroviral transduction efficiency. (A) Genomic DNA from PIDE::MLL·AF4- and PIDE::AF4·MLL-transduced BA/F3 cells was isolated and diluted to obtain 1 to 10 000 diploid genome copies. Simultaneously, PIDE::MLL·AF4 and PIDE::AF4·MLL plasmid copies (1 to 10 000) were used to detect MLL·AF4 and AF4·MLL sequences by PCR experiments. Direct comparison revealed that approximately 1 in 10 000 BA/F3 cells were transduced with the MLL·AF4 transgene, whereas 1 in 1000 BA/F3 cells were transduced with the AF4·MLL transgene. M indicates DNA size marker (lambda DNA, ClaI digested). (B) Long-range PCR was performed with infected BA/F3 cells (A: MLL·AF4, B: AF4·MLL, C: MLL·AF4 and AF4·MLL). The indicated plasmid dilutions were amplified in parallel. For the MLL·AF4 transgene, specific oligonucleotides were used that bind to MLL exon 3 and AF4 exon 20. The AF4·MLL transgene was amplified using oligonucleotides specifically binding to AF4 exon 1b and MLL exon 37. The sizes of both amplification products are indicated. M indicates DNA size marker (1-kb ladder); N, negative control.