Schematic representation of the gene structures, major splicing isoforms of human and murine Runx1 genes, and their effects on expansion of hematopoietic stem cells. Exons are indicated by boxes. The vertical dashed line within exon 6 indicates an internal splice site. Exons constituting the runt domain are indicated in gray. The two alternative promoters are denoted as P1 and P2. Crosshatched boxes indicate 5′- and 3′-untranslated regions. The coding region in the extended exon 6 is highlighted in red.