RUNX1 belongs to the runt domain family of transcription factors and regulates target gene expression through forming a heterodimeric DNA-binding complex with CBFB. RUNX1 was first identified as a RUNX1-ETO fusion protein in acute myeloid leukaemia (AML) and is frequently mutated in AML and myelodysplastic syndrome due to chromosomal translocation. RUNX1-deficient mice fail to generate hematopoietic stem cells. RUNX1 regulates CD4 gene transcription during multiple stages of T cell development and represses the CD4 gene in CD4-CD8- (double negative) T cells. RUNX1 is also required for the differentiation of CD8+, Th17, and regulatory T cells. Impaired differentiation of megakaryocytes and decreased circulating platelet count was observed in the absence of RUNX1. These findings revealed that RUNX1 acts as a tumor suppressor for myeloid leukemia and is crucial for the development and terminal differentiation of several blood cell lineages.