UASt regulatory sequences drive expression of mRFP1 tagged with Tag:MCP. A Tag:NLS(SV40-largeT) nuclear localization signal is present at the N-terminal end of the open reading frame. In the presence of a mRNA tagged with Tag:MS2 (an RNA stem loop to which Tag:MCP binds), the Tag:MCP-mRFP1 fusion protein can bind to this tagged mRNA, allowing its localization within the cell to be studied. In the absence of a Tag:MS2-tagged mRNA (or when unbound), the Tag:MCP-mRFP1 fusion protein is targeted to the nucleus due to the presence of the Tag:NLS(SV40-largeT) tag.