Sequencing of multiple neighbouring serial sections from a non-metastasized colon tumour detected ubiquitous mutations in APC and TP53 as well as copy number gains in the GNAS and EDEM2 genes on the long arm of chromosome 20 and consistent loss of heterozygosity of TP53. This analysis provides evidence for the existence of a common dominant ancestor clone proposed earlier28. Here, GNAS is linked to colonic neoplasm.