Dual and multicolor interphase FISH for the detection of breakpoints in the ALK and CLTC loci. (A) Ideograms of chromosomes 2 and 17 showing the ALK(2p23) and CLTC(17q23) flanking probes labeled with different dyes. The schematic interphase nucleus displays a FISH signal pattern indicating intact ALK and CLTC loci. (B) Scheme of chromosomes 2 and 17 containing the t(2;17)(p23;q23) reciprocal translocation as well as an interphase nucleus harboring this translocation. Derivative chromosomes 2 and 17 show a colocalization of green/pink and pale blue/red signals, respectively. The latter points to the CLTC-ALK fusion leading to activation of the ALK tyrosine kinase. (C) Interphase FISH in case 2 hybridized with the LSI ALK probe. Arrows point to tumor cells with isolated red and green signals indicating the presence of a breakpoint in the ALK locus. Arrowheads point to nontumoral cells containing the signal constellation for 2 intact ALK loci. (D-E) Interphase cells from case 3 (D) and case 4 (E) hybridized with the dual-color CLTC break-apart probe. Both cases show a split of the red and green signals pointing to a breakpoint in the CLTC locus. (F-G) False color display of interphase cells from case 2 (F) and case 1 (G) hybridized with the multicolor break-apart probe for ALK and CLTC (described in panels A-B). Both cells contain red/pale blue colocalizations, which indicate the fusion of the 5′-end of CLTC with the 3′-end of ALK leading to activation of the ALK tyrosine kinase. Besides, the presence of isolated green and pink signals indicates that the reciprocal 5′ALK-3′CLTC fusion is missing, probably due to complex chromosomal aberrations. The lack of an isolated red 3′ALK signal renders an additional fusion other than CLTC-ALK highly unlikely.