In cases of cancer that has spread throughout the body, it can sometimes be difficult to determine where the cancer originally developed. A large new study found that in such cases, molecular profiling of the cancer tissue, even if it is not from the original (“primary”) tumor, can meaningfully guide the choice of treatment. Such profiling identified useful biomarkers in 77% of the examined patients, including various genetic mutations, along with over- or underexpression of different proteins. These biomarkers then informed the selection of targeted therapies, which were highly effective in many cases. Indeed, determining the molecular profile of the cancer tissue appeared to be more useful than finding out which body part the cancer originated from.