Chronic myelogenous leukemia (CML), also called chronic myeloid leukemia, is caused by faulty instructions inside the blood stem cells, which causes them to not work properly. Every cell in the body has a set of instructions to make everything it needs. The instructions are divided into books called chromosomes and each page in a book is called a gene.
Each stem cell has a switch that controls when it divides and what sort of blood cell it divides into. In CML, the gene for making switches has partly been swapped with a gene from another chromosome. That means the instructions for making switches are wrong. When the switch is broken, stem cells make too many of the wrong type of white blood cell.