DNA damage followed by ineffective repair has been linked to several types of cancer.

Cancer cells have acquired mutations or damage to their DNA that permits them to bypass the normal limits of cell division. They become immortal and reproduce infinitely as tumor cells. Cancer-causing DNA damage also includes the loss of genes that act as tumor suppressors and the activation of oncogenes, which promote cancer.¹¹

Some cancers run in families. And some of these inherited cancers can be traced to a DNA error called base-pair mismatch, in which the subunits of DNA get lined up improperly. Normal cells have efficient mismatch repair systems; a few of us inherit defective genes for mismatch repair, and thus are prone to certain cancers.¹² This is one of nature's ironies: an inherited type of DNA damage paves the way for later acquired DNA damage and cancer.

But not all cancers arise from DNA damage that we have at birth. Most cancers arise from the accumulated damage done by years of exposure to external toxins. The most deadly of these toxins is cigarette smoke. Cigarette smoke has been extensively analyzed and found to contain hundreds of potential cancer-causing substances. Many of these toxins damage DNA by attacking the bases, becoming what are called DNA adducts. Interestingly, cells in the lungs have been found to be less efficient at repairing and removing DNA adducts than other cells in the body.¹³ This allows the number of lung cells with DNA adducts to rise, and may eventually lead to cancer.