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Hematologists study and/or treat diseases of the blood and the blood-forming tissues. Some hematologists are physicians (medical doctors, or M.D.'s) who specialize in blood diseases; other hematologists are medical scientists (doctors of philosophy, or Ph.D.'s) who do research on blood diseases but do not treat patients. A few students who plan to focus on medical research choose to do a joint M.D./Ph.D. program.