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Some cases of chronic myelomonocytic leukemia (CMML) are linked to cancer treatment, but in most cases the cause is unknown.

Over the past few years, scientists have made progress in understanding how certain changes in the DNA of bone marrow cells may cause CMML to develop. DNA is the chemical that carries the instructions for nearly everything our cells do. We usually look like our parents because they are the source of our DNA. However, DNA affects more than the way we look.

Some genes (parts of DNA) control a cell’s growth and division process. Genes that promote cell division are called oncogenes. Other genes called tumor suppressor genes can slow down cell division or even make cells die at an appropriate time. Cancers can be caused by DNA mutations (gene defects) that turn on oncogenes or turn off tumor suppressor genes.

In some diseases, mutations may be passed down from a parent. Inherited mutations do not seem to cause CMML. Instead, the mutations are acquired during the person’s lifetime. Exposure to radiation or cancer-causing chemicals can cause mutations that lead to CMML. Sometimes these gene changes occur for no apparent reason. Every time a cell prepares to divide into 2 new cells, it must copy its DNA. This process is not perfect, and copying errors can occur. Fortunately, cells have repair enzymes that read and fix DNA. However, some errors may slip past, especially if the cells are growing rapidly.

Human DNA is packaged in 23 pairs of chromosomes. In up to half of patients, CMML cells contain altered chromosomes. Sometimes part of one chromosome attaches to a different chromosome. This is called a translocation. Like mutations, translocations can turn on oncogenes or turn off tumor suppressor genes. Acquired translocations are seen in some cases of CMML. Another chromosome abnormality that can be seen in CMML is called a deletion. This involves the loss of all or part of a chromosome. Another type of chromosome abnormality is called a duplication. This is when there is an extra copy of all or part of a chromosome.