AML1/Runx1 binds DNA as a monomer and through the Runt domain. DNA binding is increased by heterodimerization with CBFB. Isoform AML1L can neither bind DNA nor heterodimerize and interferes with the transactivation activity of AML1/Runx1. CBF binds to the core site, 5'-PYGPYGGT-3', of a number of enhancers and promoters, including murine leukemia virus, polyomavirus enhancer, T cell receptor enhancers, LCK, IL3 and GMCSF promoters. The alpha subunit binds DNA and appears to have a role in the development of normal hematopoiesis. AML1/Runx1 is expressed in a wide variety of tissues and is expressed at the highest levels in thymus, bone marrow and peripheral blood. Defects in AML1/Runx1 are the cause of familial platelet disorder with associated myeloid malignancy, an autosomal dominant disease characterized by qualitative and quantitative platelet defects, and propensity to develop acute myelogenous leukemia.