In this study, Tomasetti and Vogelstein analyze the lifetime risk for different tumor types. Compiling published data for 31 different tissues, the authors find a strong correlation between tumor incidence and the number of stem cell proliferation cycles. For example, melanocytes that are known to have lower stem cell renewal rates than basal cells are significantly less likely to develop skin tumors. The authors therefore conclude that random replication errors are the major source of cancer-causing mutations. They estimate that about 65% of the differences in lifetime tumor risk are due to the number of stem cell divisions that a particular tissue type normally undergoes. They further introduce a new tumor classification that distinguishes R- from D-tumors. R-tumors accumulate mutations primarily during replication, whereas D-tumors have specific determinants, such as environmental causes or genetic predispositions that drive cancer development. These results highlight the predominant role that stochastic mistakes during DNA replication play in tumor formation.