Abnormalities in DNA in somatic cells (all the cells except sperm and egg cells) influence significantly on organism function and can have serious consequences. Tumours are diseases of genetic material resulting in uncontrolled cell division. Overcoming tumorous cells is due to immune system. It is assumed that they normally appear sometimes but are efficiently destroyed. Disease develop when organism can no longer handle growing number of cells that keep dividing in unregulated manner. Malignant tumours destroy nearby tissues and can give metastasis to other parts of the body. Tumours that are not malignant do not affect other tissues and usually don’t give metastasis, they are removed surgically.

Vast majority of cancers are not hereditary, like other somatic cell mutations. They are cause by environmental carcinogenic agents (such as cigarette smoking, UV and ionizing radiation) that most often are also mutagenic. However in some cases susceptibility to cancer, especially when it develops earlier than usually and at multiple sites, is inherited. For example the first mutation is inherited in a recessive fashion. Each cell contain only one functional copy and thus mutation in any cell can leads to disease development. Most often altered genes belong to one of three groups: protooncogenes or anti-ocogenes (both regulate cell divisions) or genes that influence DNA repair.