Detection of MLL-AF4 fusion transcripts in normal hematopoietic cells by Southern blot analysis. (A) PCR products from normal infant and fetal BM. (B) PCR products from fetal liver. PCR products were transferred to nylon membranes and hybridized as described in Materials and Methods with ³²P-labeled oligonucleotide detection probes for MLL-AF4 andBCR-ABL. Amplified mRNA from the RS4;11 and patient UPN100, with t(9;22)⁺ ALL, were used as positive controls (Pos Con) for MLL-AF4 and BCR-ABL fusion transcripts, respectively. Negative controls (Neg Con) were PCR products from RNA-free reaction mixture 1 plus reaction mixture 2 (negative control 1) and reaction mixture 1 plus DNA polymerase-free reaction mixture 2 (negative control 2). Gest., gestational.