(A) Diagram of the MLL gene. Vertical lines and open boxes represent exons. Shaded box represents 3′ UTR. Arrows represent oligonucleotides used to screen the PAC library (A and B) or used to map the PAC (L1, L27, L5, and L6). (B) Three PACs isolated which span the gene. Solid lines indicate mapped regions of the PACs, dashed lines indicate unmapped regions of the PACs. (C) Subcloned regions of PACs containing repetitive elements. Open boxes represent exons. (▿), Positions of small repetitive elements. E, EcoRI; X, Xho; T, Taq I. *No restriction digestion at this site due to overlapping Dam methylation. cen, centromere; tel, telomere; bcr, breakpoint cluster region.