Retroviral mutagenesis has been used as a powerful tool to discover genes involved in oncogenesis through a technique called Common Insertion Site (CIS) analysis where tumors are induced by proviral integrations and the genomic loci of the proviruses are identified. A fundamental assumption made in this analysis is that multiple proviral insertions in close proximity occurring more frequently than would be predicted randomly provides evidence that the genes near the integrations are involved in the formation of the tumors. We demonstrate here using data derived from MLV integrations not put under selection for tumor induction that CIS analysis as currently defined is often not a sufficient argument for a gene's significance in tumorigenesis.