Many people with Ph+ CML do not have any symptoms of the disease. Indeed, one of the most common signs of the disease is a high white blood cell count—which may be found during a routine blood test.

You’re not born with Ph+ CML. It’s also not handed down from one generation to the next. So what causes the disease?

Ph+ CML has to do with a change in chromosomes in your body. As your cells wear out, they make copies of themselves. Each cell copies everything inside it, including its chromosomes. The cell then splits in 2—creating 2 identical cells. Although it’s not known why, sometimes a mistake happens when the cell is copying itself. For example:

A piece of 1 chromosome in a cell may break off and attach to another chromosome or pieces from 2 different chromosomes may swap places. In Ph+ CML, pieces from chromosomes 9 and 22 (in humans, each cell has 23 chromosome pairs) trade places. This creates a new abnormal chromosome 22—called the Philadelphia chromosome.

Think of the BCR-ABL protein like a light switch:

The BCR-ABL protein “turns on” the bone marrow. The bone marrow starts making too many immature white blood cells. These immature white blood cells grow abnormally. Your doctor may call them leukemic cells. The immature, leukemic cells do not become healthy white blood cells

Here's what happens inside your body when you have Ph+ CML:

The leukemic cells start to grow and divide. They build up in the bone marrow, move into the bloodstream, and travel throughout the body. Over time, excess leukemic cells crowd out healthy white blood cells, red blood cells, and platelets. This can cause problems such as anemia, bruising easily, bleeding that takes longer to stop, and a greater chance of infections