Fluorescein methotrexate is useful for studying anti-metabolite resistance and spontaneous gene amplification. The dye is actively taken up by cells and binds to dihydrofolate reductase (DHFR) quantitatively.     λEx/λ Em = 496/516nm   Yellow solid soluble in pH>6.5 buffer, or DMF   Store desiccated at 4°C and protect from light   C46H54N14O9S   MW:979.08