MMP9 and breast cancer: Our first aim was to screen for novel proteases that are coregulated with ERα in breast cancer by mining the microarray dataset published by van't Veer et al. [21] Proteases including CTSC (cathepsin C), CTSF, CTSL, CTSS, CTSL2, MMP-1 (matrix metalloprotease-1), MMP-7, MMP-9, MMP-12, MMP-24, and PRSS23 that were associated with ESR1(mRNA of ERα) expression.