BCA, fully named Bicinchoninic Acid Assay, is a protein quantification method based on the biuret reaction principle. Under alkaline conditions, protein peptide bonds reduce Cu2+ (from copper sulfate) to Cu+. Subsequently, BCA chelates Cu+ to form a purple-colored complex, which strongly absorbs light at a wavelength of 562 nm. The intensity of the colored complex is proportional to protein concentration, as the reduction of Cu2+ or the amount of Cu+ formed directly reflects the amount of protein present in the solution. Thus, protein concentration in an unknown sample can be determined by measuring absorbance. This assay kit features easy operation, high stability, high sensitivity and excellent compatibility. It can detect protein concentrations as low as 25 μg/mL, with a minimum detection limit as low as 0.5 μg protein. It requires a sample volume of only 1–20 μL. The assay shows good linearity in the concentration range of 50–2000 μg/mL.