Dideoxy DNA sequencing is routinely used in research and, increasingly, in clinical care for the detection of DNA sequence variants, single nucleotide changes, or small insertions or deletions, when the spectrum of DNA variation is unknown. DNA sequence variation can be present in tumor tissue that is not present in the normal tissue from the same individual. This somatic DNA sequence variation is often the cause of abnormal cell growth and/or regulation and, ultimately, tumorigenesis. Identification of these oncogenic DNA sequence variants has successfully led to the development of cancer therapies, since the abnormal protein products created from genomic DNA containing mutations can serve as targets for pharmacologic inhibition. Somatic DNA sequence analysis will continue to be a valuable technique for biomarker discovery until the complete spectrum of DNA variation observed in tumor tissue is understood.