We have dissected tissue from P3 mice stably expressing a protein tagged with mCherry and fixed it for 4 hours in 4% PFA (pH 7.3) at 4°C. tissue is then frozen in OCT and then cut. and mounted on gelatin coated slides.The fluorescence is bright when first viewed (immediately after sectioning); however when the sections are gently washed in ddH2O and then mounted in aqueous mounting media such as FluoromountG or Prolong Gold, the fluorescence disappears. Is this to do with the photostability of mCherry over time?