Fluorescence in situ hybridization. FISH with a home-brew dual color break-apart probe set for RET gene (5' labelled with Spectrum Orange and 3' labeled with Spectrum Green). Cells with unrearranged RET show intact red/green signals (one fused signal for each RET gene); cells with rearranged RET show probe signal split into its 5' (red signal) and 3' (green signal) portions (two discrete red and green signals for each rearranged RET gene). NTHY-ORI 3-1 representative nuclei with four (a, top) and two (a, bottom) copies of intact RET; TPC1 representative nucleus with two copies of intact RET (two red/green signals) and two copies of rearranged RET (two discrete red and green signals) (b); K1 representative nucleus with four copies of intact RET (c); B-CPAP repesentative nucleus with three copies of intact RET (d). Nuclei are counterstained with 4',6-diamidino-2-phenylindole.