Congo Red (Direct Red 28) is an azo dye. It binding been used as a diagnostic test for the presence of amyloid in tissue sections.

Congo Red histochemical stain may serve as a tool to investigate if aggregates in mutant cells have misfolded β-pleated sheet secondary structures [1]. Congo Red dye specifically binds to crossed β-pleated sheet structures. While wild-type HSPB1 maintains protein homeostasis by binding proteins in non-native conformations and preventing aggregation, the T139M mutant fails in this function, resulting in the accumulation of misfolded proteins targeted by Congo Red for intercalation between the β-pleated sheet structures.