Jäger et al. examined whole cancer genomes of 402 samples of diverse cancer types and discovered that the inactive X chromosome (Xi) in the tumours of female cancer patients has a high rate of somatic mutation. Hypermutation of Xi seemed to be cancer-specific, and hypermutation was not observed on the active X chromosome. Furthermore, Xi had a higher mutation rate than autosomes, and various analyses showed that hypermutation of Xi occurs early in tumorigenesis and by the same mutational processes that lead to autosomal mutations.