Tag:MCP represents the coat protein of the MS2 RNA bacteriophage. It specifically binds a 19bp RNA stem loop structure that is represented in FlyBase by the Tag:MS2 entry. Tag:MS2 and Tag:MCP thus form a binary RNA-protein interacting pair which are typically used to either monitor or alter the localization of an mRNA of a gene of interest, depending on what other sequence the Tag:MCP component is fused to. For example, if a transgene or modified endogenous locus in which the Tag:MCP protein is fused to a fluorescent protein is combined with a second transgene or modified endogenous locus encoding an mRNA of interest tagged with the Tag:MS2 stem loop, the localization of this tagged mRNA can be followed in vivo. Conversely, if the Tag:MCP protein is fused to sequence that targets it to a particular subcellular location, it be used to target an mRNA of interest tagged with the Tag:MS2 stem loop to that location, so that the effect of altering the location of the tagged mRNA can be studied.