Formalin-fixed paraffin-embedded tissue (FFPET), which is the most widely used pathology archive, usually has low-quality DNA and RNA due to extensive nucleic acid crosslinking. RNA fluorescence in situ hybridization (RNA-FISH) has been increasingly utilized in research and clinical settings to diagnose disease pathology. In this study, the effect of RNA degradation over archival time on RNA-FISH signals in FFPET and fresh frozen tissue (FFT) was systematically assessed. RNAscope multiplex fluorescent assay with the four house-keeping-gene (HKG) probes