(A) Southern blot analysis to assess proviral integrity and copy number in cells genetically modified with the MGirL22Y retroviral vector. Genomic DNA from cells was restricted with Asc I which cuts once within each viral LTR, liberating the full-length, 3.6-kb retroviral genome. DNAs analyzed (15 μg) in each lane are: 1, GP + E86 DNA containing 50 pg of the MGirL22Y vector plasmid DNA; 2, GP + E86; 3, GP + E86-derived MGirL22Y viral producer; 4, NIH 3T3; 5, GFP vector-transduced NIH 3T3; 6, CD24 vector-transduced BM; 7, GFP vector-transduced BM. The migration patterns of the molecular-weight markers are shown at left. (B) Schematic of the MGirL22Y retroviral vector which consists of the MSCV retroviral backbone containing the coding sequence of GFP followed by an internal ribosomal entry site (IRES or ir) from the encephalomyocarditis virus linked to a mutant dihydrofolate reductase gene (L22Y). The designated probe region depicts the fragment used in the Southern analysis.