Subtle changes in cell mass can be used as a functional biomarker to predict patients’ response to cancer drugs, a new retrospective study on glioblastoma patient-derived neurosphere models reports. This functional precision medicine approach where you take a patient’s tumor cells, give them the drugs that the patient might get, and predict the outcome before giving them to the patient, could potentially match patients to the most effective available chemotherapy drugs and is particularly helpful for cancers and drugs that lack genomic biomarkers.