MTSEA biotin selectively labels thiols under mild conditions. We offer MTSEA biotin with different linker lengths (Biotin-MTSEA, Biotin-X-MTSEA, and Biotin-XX-MTSEA), all of which allow selective labeling of cell surface thiols. Longer linkers should facilitate interaction between biotin and streptavidin. MTSEA biotin-X