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The mirror protease only works on mirror peptides, which means, by the law of mirror-image symmetry that applies to chiral molecules, that regular proteases would likewise be unable to cut down mirror-image peptides.

"Even the simplest cells contain proteases and nucleases and regularly degrade and replace their proteins and RNAs, indicating that SAI is essential for life," he explains.

Their report, which will appear in Science, shows that an alternative medication, a viral papain-like protease inhibitor, inhibits disease progression in animals, a necessary step before human drug trials.

These sensors consist of polymer nanoparticles coated with a reporter, such as a DNA barcode, that is cleaved from the particle when the sensor encounters enzymes called proteases, which are often overactive in tumors.

If the desired protease is present in its active form in the sample, the peptide splits it.