As fundamental green-emitting fluorophores (λex/em ~495/520 nm), fluorescein derivatives remain cornerstone tools in biological detection. Their high-water solubility, substantial quantum yield (~0.9), and well-characterized chemistry make them ideal for standardized applications. Featuring reactive moieties (isothiocyanate, NHS ester), they readily conjugate to antibodies and oligonucleotides. While moderately susceptible to photobleaching, their cost-effectiveness and consistent performance sustain widespread use in cellular imaging, ELISA, immunohistochemistry, and lateral flow assays where extreme photostability isn't critical.