Both of these images show human lung tissue that has been stained with activatable zymography probes (AZPs). These probes are designed to mark sites of protease cleavage with a fluorescent label. Because proteases, which are enzymes, are involved in a wide range of biological processes, these probes can be used to detect areas of upregulated biological activity, which is a hallmark of cancer and other diseases. Here, the probes are detecting pulmonary protease activity. By using AZPs to localize protease activity, we can learn about the biological processes underlying disease, and even create new methods to diagnose diseases from tissue samples.