The immune system is less likely to attack tumours that have an abnormal number of chromosomes, which could be one reason why cancer immunotherapies work well in only a fraction of people.

Many types of tumour have too many or too few chromosomes — a condition called aneuploidy. Stephen Elledge at Harvard Medical School in Boston, Massachusetts, and his colleagues looked into whether abnormal numbers of chromosomes and chromosome fragments can predict response to drugs called checkpoint inhibitors, which can unleash an immune response against cancer.

A search through genome sequences from more than 5,000 human tumours representing 12 cancer types showed that those with a high degree of aneuploidy tended to contain fewer immune cells. Data from two clinical trials also showed that people with melanoma were less likely to respond to checkpoint inhibitors if their tumours had higher levels of aneuploidy.

Science 355, eaaf8399 (2017)