(A) MLL and AF9 break points in UG3 cells. Break points in UG3 cells as determined by sequencing the RT-PCR products are shown with vertical black arrow heads. Primers are shown as horizontal arrow heads; ex, exon; bps, base pairs. (B) RT-PCR analysis for AF9-MLL and MLL-AF9 fusion transcripts in UG3 cells. RT-PCR analysis was performed as detailed in the Materials and Methods. Amplification reactions performed with all primer combinations failed to produce any PCR product, with the exception of the following primer pairs: AF9s-MLLex12as (lane 1), AF9s-MLLex14as (lane 3), and MLLex9s-AF9as (lane 5). As a control, RT-PCR analysis was also performed with reversely transcribed RNA obtained from K562 cells using these primer pairs, and reaction products were loaded into lanes 2, 4, and 6, respectively. Lane M depicts pBR322 DNA digested with Msp I as a molecular weight marker. Shown is one representative result of four independent experiments.