Scientists have discovered a rare "mirror image" version of the amino acid cysteine, called D-cysteine, that slows the growth of some cancers. This molecule is mainly taken up by cancer cells through a specific transporter on their surface. Healthy cells remain largely intact. Once inside cancer cells, D-cysteine shuts down a key mitochondrial enzyme. This enzyme is needed to produce energy and maintain DNA in cancer cells. As a result, their growth is effectively stopped. Unlike conventional anti-cancer treatments, this molecule does not damage normal tissues.