MoonTag is a fluorescence labeling system designed to visualize translation of single mRNAs in real time. It enables the study of translational heterogeneity by revealing how different polypeptides can be produced from a single mRNA. When used alongside the orthogonal SunTag system, MoonTag allows dual-color tracking of translation from different reading frames, providing powerful insight into start site selection dynamics. Its high sensitivity, multiplexing capability, and real-time imaging make it a valuable tool for investigating complex translation behaviors in living cells.