FISH analysis of IKZF1 deletions. (A) Map of the deletion proximal breakpoints within the IKZF1 gene is shown. Purple arrowheads indicate the breakpoint regions mapped by FISH experiments. Particularly, the left arrow corresponds to both Δ4-7 and Δ2-7 deletion proximal breakpoint regions, whereas the right arrow indicates a novel proximal breakpoint region identified in patient #17 (see panel B). (B) FISH results obtained in some of the cases under study showing homozygous (cases #17, #30, and #50) or heterozygous (cases #24 and #28) deletion of the fosmid clone G248P800745C8 (red) only in Ph⁺ (middle column), as shown by the FISH pattern of the RP11-164N13 (BCR) probe (in yellow). Clone G248P87926C7 (green) is always retained on deleted chromosomes 7. Notably, in patient #17, 1 of the 2 green signals is fainter than the other, revealing the occurrence of a partial deletion of G248P87926C7 in 1 of the 2 chromosome 7 homologs.