developing malignant disease later in life. Though routine screening is not advised, the screening of families with a history of RB is beneficial.
Diagnosis The U.S. Food and Drug Administration's approval of the first DNA probe-based diagnostic test for chronic myelogenous leukemia launched the now rapidly expanding field of molecular diagnosis as it relates to cancer.
The Philadelphia chromosome (Ph1), a specific abnormality in this type of leukemia, was the first well-described chromosomal marker related to a gene rearrangement in cancer. The changes that result in this chromosome transforming a normal cell into a leukemic cell are now known. A proto-oncogene called c-abl normally resides on chromosome 9. It is activated—in other words, becomes an oncogene—when it travels to a critical point on chromosome 22. The exchange of genetic material between chromosomes 9 and 22 results in a shortened chromosome 22, the Philadelphia chromosome. It is believed that the activated c-abl oncogene produces an increased amount of an enzyme known as a protein tyrosine kinase that sends stimulative signals to the cell's nucleus. The result of this persistent signaling in a blood cell is development of a leukemic cell.
The importance of this genetic mechanism from a diagnostic point of view is that a DNA probe is now available that confirms the diagnosis of chronic myelogenous leukemia.
Similar DNA probes are being developed as aids in the diagnosis of different types of leukemia, lymphomas and other cancers. They appear to be able to recognize gene rearrangements and mutations in a high number of
