Abstract: The present disclosure provides tag-specific fusion proteins for selectively detecting molecules containing a strep-tag peptide or cells containing a strep-tag peptide. Disclosed embodiments include tag-specific fusion proteins that can be used in reagents and methods for monitoring and/or modulating immunotherapy cells that express a strep-tag peptide. Embodiments including fusion proteins that specifically bind tagged targets and recombinant host cells comprising polynucleotides encoding the tag-specific fusion proteins are also provided. Immunotherapy cells that express a tagged marker are also provided.