One cell at a time

Single-cell DNA sequencing of two breast-cancer types has shown extensive mutational variation in individual tumours, confirming that generation of genetic diversity may be inherent in how tumours evolve. See Article p.155 Human breast cancers often display intratumour genomic heterogeneity, making clinical diagnosis difficult and complicating the interpretation of research results. This study tackles the problem using a newly developed whole-genome single-cell sequencing technique called nuc-seq that makes use of the natural genome duplication that occurs in the S phase of the cell cycle to achieve 91% mean coverage breadth. The method is applied to sequence single normal and tumour nuclei from an oestrogen-receptor-positive breast cancer and a triple-negative ductal carcinoma. Aneuploid rearrangements emerge as early events, and they remain stable during clonal expansion. In contrast, point mutations appear to evolve gradually, generating extensive clonal diversity. The data also show that no two single tumour cells are genetically identical, raising interesting questions as to the strict definition of a clone.