CD33 and leukemia: They may be conceptually divided as follows: a. lineage-restricted antigens (LRAs), characterized by widespread expression by both leukemic and other cell types, especially myeloid progenitors and the monocyte/macrophage system (e.g., CD33, CD123, CLL-1); b. leukemia-associated antigens (LAAs), being hyperexpressed by leukemic cells but also present in other cell types (e.g., FLT3, WT1); c. leukemia-specific antigens (LSAs), highly specific to AML cells and often produced as the result of a pathogenic mutation involved in leukemic development (e.g., mutated NPM1, mutated FLT3).