Cancer genetics has traditionally focused on mutations in individual genes. A new study, however, focuses on chromosomes, the structures on which genes are located. Cancer cells often have too many or too few copies of a chromosome. This phenomenon, called “aneuploidy”, is often considered to be just a byproduct of the cancer cell’s chaotic state. In the study, researchers identified genes likely to promote or suppress tumor growth; they then noted on which chromosomes these genes were located. Chromosomes containing more tumor-promoting than tumor-suppressing genes were more likely to be multiplied inside cancer cells, while chromosomes with more tumor-suppressing genes were likely to be deleted. This finding suggests that chromosome deletions or multiplications may indeed be a cause of cancer, instead of an effect.