Interestingly, in contrast to our own observations using a luminal A MCF7 model, no reduction in tumor burden was observed with respect the MDA-MB-231 xenograft and VBY-999 treatment [19], suggesting that classical CTSS pathological roles in ECM degradation and neo-angiogenesis may be subtype specific, warranting a more in-depth analysis of breast cancer heterogeneity at the patient sample level. Here, CTSS is linked to neoplasm.