mEosEM is a monomeric fluorescent protein that can be irreversibly photoconverted from an initial green fluorescent form to a red fluorescent form upon UV irradiation. The green fluorescent form has an excitation peak of 503nm and an emission peak of 513nm, while the red fluorescent form has an excitation peak of 569nm and an emission peak of 579nm in vitro. mEosEM retains good fluorescence and photoswitching properties after osmium tetroxide treatment and embedding in Epon epoxy resin, and can be used for improved super-resolution correlative light and electron microscopy (SR-CLEM). mEosEM is an artificial derivative of the naturally occurring fluorescent protein encoded by the Lobophyllia hemprichii EosFP gene (GenBank:AY765217). It contains the mutations D28E, L93M and N166G relative to mEos4b (PMID:31611693).