Cathepsin K zymography potential as a clinical diagnostic tool for breast cancer . A) Normal and tumor breast tissue zymograms were compared for patient-to-patient variation. Standard dose curves of recombinant cathepsin K (0.2, 0.5, 1, and 5 ng) and cathepsin L (45, 220, 450, and 900 ng) were loaded per gel for quantifying absolute quantities of cathepsins K and L. B) Cathepsins K and L activity were quantified with densitometry and compared to the standard curve generated to semi-quantitatively determine nanograms of active enzyme. C) Sensitivity (blue line), specificity (red line), and likelihood ratio (dotted black line) were calculated over a range of values to identify an optimal threshold value for cathepsins K and L that would distinguish normal samples from tumor samples. Yellow boxes outline the region of maximal likelihood ratio.