Analysis of interphase nuclei using two-color NPM-ALK FISH. (A) Normal PB lymphocyte nucleus showing nonpaired, randomly spaced green and red fluorescent signals. (B) Interphase nucleus from the t(2; 5)-positive cell line SUP-M2 illustrating a yellow fusion signal produced by overlapping NPM and ALK DNA probes (upper arrow), as well as paired green and red fluorescent signals (lower arrow) produced by juxtaposition of the probes on the der(5) chromatin strands. (C) A representative interphase nucleus from the analysis of a t(2; 5)-positive clinical sample is shown (case no. 932499). Overlapping (yellow) and paired (green and red) signals (arrows) indicating the der(5) of the t(2; 5) are seen in this translocation-positive NHL sample. The nuclei in (A) and (C) were counterstained with DAPI which produces a blue background; (B) was photographed from a preparation that was not counterstained.