SPR and SPRi biosensors enable the determination in blood plasma/serum of commonly used cancer biomarkers such as cancer antigen 125 (CA 125) [11], human epididymis protein 4 (HE 4) [12], and emerging cancer biomarkers such as aromatase [13], as well as numerous other biomarkers including cathepsins B, D, and G [14]. Here, CTSB is linked to cancer.