The DNA (at least some of it) is promoter genes from the SV40 virus (Simian Virus 40) that was found in small pox vaccines (because the virus was grown in contaminated monkey cells). Many cases of cancer today have SV40 virus in them and it is believed that SV40 causes cancer. A section of genes in the SV40 virus act as promoter genes and are used in growing viruses and/or genetic substances. If these promoter genes get integrated in your DNA, they can promote other genes and make them more likely to go cancerous. I think there is other DNA as well, but the SVR40 promoter genes are what is mostly being discussed.