This event is the alteration of a gene on the cells of a specific tissue causing the gene to become a cancer-causing gene or oncogene. It is called "somatic" to distinguish it from a germ cell mutation, which can be passed from parent to offspring. Most cases of leukemia are caused by somatic mutation in a primitive marrow (blood-forming) cell. If the mutation results from a major abnormality of chromosomes such as a translocation, it can be detected by cytogenetic examination. Often the alteration in the gene is more subtle and requires more sensitive tests to identify the oncogene.