MCPyV detection by FISH in previously reported MCC and CLL cases.^1,4  MCPyV FISH was carried out using tyramide signal amplification (TSA) as described previously.^3,5  Signal evaluation was performed with a Leica DM 5000 B fluorescence microscope (Leica) using 1000× magnification and specific filter sets for Texas red to visualize MCPyV (A,C,E,G) and DAPI showing blue nuclear counterstaining (B,D,F,H). (A-B) MCPyV detection in MCC and corresponding DAPI (B). (C-D) Negative control on MCC tissue without probe and corresponding DAPI (D). (E-F) MCPyV detection in CLL cells of a bone marrow trephine and corresponding DAPI (F). (G-H) MCPyV detection in another CLL case in a bone marrow trephine and corresponding DAPI (H).