Mutated clones are the new normal

Measuring and understanding the dynamics of clonal cell populations is key for cancer prevention Cancers are formed by the expansion of harmful “mutational clones,” which are cell populations carrying the same DNA mutations. How harmful they are depends on which mutated genes they contain. On page 970 of this issue, Yizhak et al. ( 1 ) add to the evidence that normal tissues are not so normal after all. Examining a substantial number of healthy tissues from almost 500 individuals, they found a large number of acquired (somatic) DNA mutations—some of which are typically associated with cancer—and mutational clones of macroscopic size, in the absence of cancer pathology. The presence in normal tissues of clonal cell populations, with mutations in cancer-associated genes, is informative about how a tumor evolves from the first mutation to a benign growth and, finally, to cancer.