While some people with acute lymphocytic leukemia have one or more known risk factors mentioned earlier, most do not. The cause of their cancer remains unknown at this time. Even when a patient has one or more risk factors, there is no way to tell whether it actually caused the cancer. And many people with one or more cancer risk factors never develop this disease.

During the past few years, scientists have made great progress in understanding how certain changes in DNA can cause normal bone marrow cells to become leukemic cells. DNA is the chemical that carries the instructions for nearly everything our cells do. We usually resemble our parents because they are the source of our DNA. However, DNA affects more than our outward appearance.

Some genes (parts of our DNA) contain instructions for controlling when our cells grow and divide. Certain genes that promote cell division are called oncogenes. Others that slow down cell division or cause cells to die at the appropriate time are called tumor suppressor genes. We know that cancers can be caused by DNA mutations (gene defects) that turn on oncogenes or turn off tumor suppressor genes.

Every time a cell prepares to divide into two new cells, it must duplicate its DNA. This process is not perfect and copying errors can occur. Fortunately, cells have repair enzymes that proof read DNA. But some errors may slip past, especially if the cells are growing rapidly.

Translocations are the best known type of DNA abnormality that can cause leukemia to develop. Human DNA is packaged in 23 pairs of chromosomes. A translocation means that DNA from one chromosome breaks off and becomes attached to a different chromosome. Translocations, which often occur in cases of ALL, can cause oncogenes to be turned on. The most common translocation seen in ALL is known as the "Philadelphia chromosome," which is a translocation between chromosomes 9 and 22, abbreviated as t(9;22). It occurs in about 20%-25% of cases of ALL. Other, less common, translocations are those between chromosomes 4 and 11, abbreviated as t(4;11), or 8 and 14, abbreviated as t(8;14). Other chromosome changes such as deletions (the loss of part of a chromosome) and inversions (the rearrangement of the DNA in part of a chromosome) can also affect the development of ALL, although they are much rarer.

Some people with certain types of cancer have inherited DNA mutations from a parent. These changes increase their risk for the disease. However, ALL is very rarely caused by one of these inherited mutations.

Usually DNA mutations related to ALL occur during life rather than having been inherited before birth. Acquired mutations may result from exposure to radiation or cancer-causing chemicals. Sometimes they occur for no apparent reason.

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