Tumors change over time. Abnormalities in chromosomes are a main cause for these progressive changes in the nature and composition of cells that make up a tumor. These changes can lead to therapy resistance in for instance glioblastoma, the deadliest brain cancer. Biophysicists develop a technology to identify rare cancer cells bearing severe chromosomal abnormalities and to study these cells at unprecedented resolution. They investigate the causes and consequences of abnormal chromosomes in glioblastoma and aim to generate information that can lead to improved treatment for glioblastoma.