CTSB and breast cancer: Although it has been shown that cathepsin B, L and S have elevated expression levels and enzymatic activity in breast cancer and that this upregulation correlates with an increased imaging agent activation, the association between their upregulation and relevant clinicopathological characteristics of breast cancer patients, such as histology, grade, molecular subtype and neoadjuvant treatment status, is not well defined and has only been investigated in relatively small patient cohorts[15–17, 20–23].