Substances that act as reducing agents (e.g. dithiothreitol or 2-mercaptoethanol) can alter the assay's chemistry and lead to inaccurate protein measurements. Acidifiers, reducing sugars, copper chelating agents, lipids, and phospholipids can also all interfere with the assay and impact the results. Additionally, amino acids like cysteine, tyrosine, and tryptophan can produce color changes that might interfere with the measurement of proteins by contributing additional color. Another factor that can affect the accuracy of the BCA assay is the presence of chemically modified residues in the protein. For example, researchers found that in the BCA assay, the protein concentrations in samples with methylated lysine residues were consistently overestimated.