Hematopoietic and endothelial cells arise from a common progenitor cell called a hemangioblast. Hemangioblast specification occurs early during the process of mesoderm development and is promoted by transcription factor expression. Gain- and loss-of function studies in animal model systems have implicated the transcription factor LMO2 in the process of hemangioblast specification. In addition, embryonic stem cells lacking RUNX1/CBFA2 produce reduced numbers of hemangioblasts, indicating that it plays an important though non-essential role during hemangioblast development. Transcription factors also play important roles in hemangioblast differentiation. For example, SCL/Tal1 expression is thought to generate hematopoietic cells from hemangioblasts.