The level of several matrix-associated proteins was increased in the conditioned medium from MDA-1833 spheroids cultured in our metastasis-on-a-chip platform, namely connective tissue growth factor (CTGF) and matrix metalloproteinase 1 (MMP1), which were already described to promote breast cancer progression in the bone niche (Fig. 2f) [31,[35], [36], [37]]. Here, MMP1 is linked to breast cancer.