Extended Data Figure 8 |. Melanocytes accumulate few mutations in tissue culture.

a, We sequenced a bulk culture of neonatal melanocytes to establish the germline SNPs and somatic mutations in the dominant clones. We continued to passage the cell line for 239 days, genotyping individual clones at the timepoints indicated to establish the rate at which mutations were acquired in culture. In parallel, Petljak et al¹⁸ performed similar experiments on common cancer cell lines, and we analysed their data from a melanoma cell line (Mewo) included in their study. b, On average, the mutation burden of neonatal melanocytes and Mewo cells respectively increased by 0.090 and 0.086 mutations/Mb for every 2 weeks in tissue culture (we typically cultured melanocytes 2 weeks or less in this study). To put these mutation burdens in perspective, the average mutation burdens of sun-exposed and sun-shielded melanocytes from this study are shown in comparison. Based on these results, we conclude that the brief period of tissue culture contributed little towards the mutation burdens observed in our study.