Analysis of the MLL locus by FISH in patient B22. An MLL locus specific probe was hybridized to cells from bone marrow aspirates taken at presentation. Chromosomes were stained with DAPI. (A) Normal interphase cell with two MLL alleles. (B) Interphase cell with one MLL allele deleted. (C) Normal metaphase cell with two MLL alleles. (D) Metaphase cell with one MLL allele deleted.