Domain architect of RUNX1 protein and its role as a transcription factor. (A) A schematic depicting the 3 major isoforms of RUNX1 (1A, 1B, and 1C). Isoforms 1A and 1B are transcribed from P2, and isoform 1C is transcribed from P1, thus differing by 32 amino acids at its 5′ end (marked in orange). Isoform 1A contains only the RHD and differs by 9 amino acids at its 3′ end (marked in orange). (B) Schematic of the protein encoded by the largest isoform, 1C, with major functional domains marked: RHD and transactivation domain (TAD). The numbers above the lines represent the amino acid residues. (C) A schematic of RUNX1 heterodimerization with its binding partner, CBFβ, and interaction with DNA at promoters of target genes that carry the specific binding site: YGYGGTY, where Y is C or T.