We analyzed FFPE-post-mortem tissue biopsies from 3 ME/CFS patients and three patients with other clinical diagnosis (anorexia, non-Hodgkin’s lymphoma, and breast cancer) for potential signature of HHV-6 infection by immunofluorescence analysis using antibodies against HHV-6B U94 (potential marker for viral latency), HHV-6B late protein OHV-3 (marker for active infection) and HHV-6 glycoprotein gB and p41 (markers for active infection). Here, ART4 is linked to breast carcinoma.