To Quote The Credible Hulk on FaceBook
DNA polymerase adds about 1 wrong deoxyribonucleotide per 100k bases during DNA replication. It can also “proofread” and correct many such mistakes, resulting in an error rate around 1 in 10 million bases in eukaryotes. A battery of mismatch repair enzymes edit remaining errors after replication is complete, bringing the error rate to about 1 in a billion. That’s accurate, but not accurate enough considering how many times DNA has to be replicated. This is in addition to induced mutations from environmental carcinogens. If some of these mutations disable some of the regulatory control mechanisms of the cell cycle, a cell may continue replicating when it shouldn’t. – Cred Hulk
We usually attribute cancer to inherited genes such as the BRACA1 or BRACA2 mutations, responsible for breast, ovarian, and prostate cancer. Or else, a lifestyle choice like smoking or something to do with the environment, say exposure to pollution or radiation.