Physical map of the PML/RAR homologous recombination vector. Genomic DNA was isolated from NB4 cells as described in Materials and Methods and detailed restriction maps were constructed in the breakpoint region 3 (bcr3) for PML and RAR, as shown in this schematic. The knockout vector was constructed using a 1.2-kb 5′ sequence homologous to PML, followed by an in-frame, promotorless, ATGless, neo gene and then by a 4.2-kb 3′ sequence homologous to RAR. The plasmid backbone is Bluescript SK+, and the vector was linearized with EcoRI before transfection into NB4-R1 cells.