DQ-BSA-Red is a bovine serum albumin (BSA) molecule labeled with a red fluorescent dye, designed for monitoring lysosomal activity. The excitation and emission wavelengths of DQ-BSA-Red are 590 nm and 620 nm, respectively. The BSA in DQ-BSA-Red is heavily tagged with the red fluorescent dye at multiple sites, leading to significant fluorescence self-quenching. Upon entry into the lysosome, DQ-BSA is cleaved by lysosomal proteases, resulting in the unquenching and release of fluorescent fragments that emit bright fluorescence. Conversely, functionally inactive lysosomes fail to degrade the BSA protein, thereby displaying lower or no fluorescence signal.