This is a diagram of normal hematopoiesis. What cell is affected in CML. As you have learned, most cancers require multiple oncogenic hits to produce a malignancy. Why does CML require only one?

CML results from the aberrant activation of a myeloid stem cell. Like most leukemias, CML requires few (or even one) mutation because it only needs to replicate within the bone marrow. It is already within the blood system and therefore doesn't need to develop the ability to spread hematogenously or live within other organs.