*2.4. KDR*/*FLK1-Expressing CPCs*

During cell movement from the primitive streak to the anterior regions of the embryo, the precardiac mesodermal cells start to express a receptor for Vascular Endothelial Growth Factor (VEGF) called KDR/FLK1 [20,81]. These FLK1-expressing progenitor cells have the ability to generate cells from both hematopoietic and cardiac lineages [20,81–83]. Selection between these two lineages is determined by the levels of FLK1 activity [81]. For example, high expression of FLK1 promotes differentiation towards hematopoietic lineages, whereas low or absent FLK1 expression stimulates cells to follow the cardiac fate [81,82]. These negative FLK1-expressing cells further generate a second FLK1<sup>+</sup> cell population that represents the first multipotent cardiac progenitor cells that are permanently committed to the cardiogenic fate [20,29]. Because KDR/FLK1 displays a broad expression, it is frequently used in combination with other cardiac markers, such as Platelet-Derived Growth Factor-alpha (PDGFRα), C-X-C chemokine Receptor type 4 (CXCR4) and sometimes MESP1/2, to enrich for CPCs [79,80].
