Malignancy is best known as a characterization of cancer. A malignant tumor contrasts with a benign noncancerous tumor that is not self-limited in its growth, is capable of invading adjacent tissues and may be able to spread to distant tissues. A benign tumor has none of these properties. Malignancy in cancer is characterized by anaplasia, invasion and metastasis. Malignant tumors are aragonally characterized by genome instability, so that cancers, as assessed by whole genome sequencing, often have between 10,000 and 100,000 mutations in their entire genome. Cancers exhibit tumor heterogeneity, with several subclones. They often have reduced expression of DNA repair enzymes due to the epigenetic methylation of DNA repair genes or modified microRNAs that control the expression of repair genes.