One of the first oncogenes to be defined in cancer research is the ras oncogene. BRAF and Ras are two oncogenes implicated in cellular senescence. L1 insertions have been associated with tumorgenesis by activating cancer-related genes oncogenes and tumor suppressors. Many cancer drugs target the proteins encoded by oncogenes. Typically, there is hypermethylation of tumor suppressor genes and hypomethylation of oncogenes. Duesberg disputes the importance of oncogenes and retroviruses in cancer. The oncogenes and tumor suppressor genes are well studied and are described above under Pathogenesis. Many important cancer-promoting oncogenes are fusion genes produced in this way. Mutations in such microRNAs (known as oncomirs) can lead to activation of oncogenes. Here, miRNAs can function as both oncogenes and tumor suppressors. Antisense messenger RNAs could theoretically be used to block the effects of oncogenes. Duesberg rejects the importance of mutations, oncogenes, and anti-oncogenes entirely. Duplications of oncogenes are a common cause of many types of cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. In general, these vaccines focus on the main HPV oncogenes, E6 and E7.