In patients with non-small-cell lung cancer, distinguishing lung metastases and independent primary lung cancers is challenging and has important clinical ramifications. Murphy et al. have now developed a DNA-sequencing-based diagnostic test that holds promise in this regard. The test identified metastases and multiple primary tumours based on the presence or absence, respectively, of shared chromosomal rearrangements, with the genomic data showing good concordance with histological findings. Importantly, no somatic chromosomal breakpoints were shared between any independent primary tumours.