Sulforhodamine B sodium salt is a fluorescent dye with uses spanning from laser-induced fluorescence (LIF) to the quantification of cellular proteins of cultured cells. In Vitro: Sulforhodamine B (SRB) is often used as a membrane-impermeable polar tracer or used for cell density determination via determination of cellular proteins (cytotoxicity assay). The SRB assay has been used to inexpensively conduct various screening assays to investigate cytotoxicity in cell based studies. This method relies on the property of SRB, which binds stoichiometrically to proteins under mild acidic conditions and then can be extracted using basic conditions; thus, the amount of bound dye can be used as a proxy for cell mass, which can then be extrapolated to measure cell proliferation. The protocol can be divided into four main steps: preparation of treatment, incubation of cells with treatment of choice, cell fixation and SRB staining, and absorbance measurement. This assay is limited to manual or semiautomatic screening, and can be used in an efficient and sensitive manner to test chemotherapeutic drugs or small molecules in adherent cells. It also has applications in evaluating the effects of gene expression modulation (knockdown, gene expression upregulation), as well as to study the effects of miRNA replacement on cell proliferation^[1].