A 45-year-old woman presents with marked splenomegaly. Her leukocyte count is increased to 300,000/uL. The differential count reveals the presence of small numbers of myeloblasts and promyelocytes, with a predominance of myelocytes, metamyelocytes, bands, and segmented neutrophils. Basophils are also increased in number, as are platelets. The patient is not anemic. Leukocyte alkaline phosphatase is decreased. Which of the following describes a major characteristic of this disorder?