Familial erythroleukemia (FERLK) is a leukemic or preleukemic state in which red cell proliferation is the predominant feature. Hematologic characteristics include particularly ineffective and hyperplastic erythropoiesis with megaloblastic components accompanied by myeloblastic proliferation of varying degree (Park et al., 2002). Park et al. (2002) discussed the evolution of the definition of 'erythroleukemia,' which is considered by most to be a subtype of acute myelogenous leukemia (AML; 601626). Controversy about the precise definition of erythroleukemia revolves around the number or percentage of erythroblasts and myeloblasts found in the bone marrow and peripheral circulation. In the French-American-British (FAB) classification system (Bennett et al., 1985), it is known as AML-M6, whereas in the revised World Health Organization (WHO) classification system (Harris et al., 1999), it is known as 'AML, not otherwise categorized' (Zini and D'Onofrio, 2004). [from OMIM]