Diagrammatic representation of two-color NPM-ALK FISH in the analysis of (A) metaphase chromosomes and (B) interphase nuclei. In (A), an idiogram of normal chromosomes 2 and 5, and the derivative chromosomes 2 and 5 produced by the t(2; 5), is shown. The location of the ALK and NPM DNA probes used for FISH in this study relative to the t(2; 5) breakpoints on the normal chromosomes 2 and 5, respectively, is indicated. Juxtaposition of the green (NPM ) and red (ALK ) fluorescent signals in a metaphase preparation identifies the der(5) chromosome, which encodes the oncogenic NPM-ALK fusion protein. In (B), the random spacing of NPM (green) and ALK (red) fluorescent signals in a normal diploid interphase nucleus is shown. In nuclei from cells containing the t(2; 5), the chromatin strands representing the der(5)-encoded NPM-ALK fusion gene are apparent as paired green and red signals or as yellow “fusion” signals resulting from overlapping green and red fluorescence.