A 4-year-old boy presents with characteristic facial features including bitemporal narrowing, periorbital fullness, a short upturned nose with flat nasal bridge, and stellate iris pattern. His parents report he is overly friendly with strangers and has a distinctive speech pattern. Echocardiography reveals a diffuse narrowing of the ascending aorta with supravalvular aortic stenosis. The diagnosis of Williams-Beuren Syndrome is suspected. The chromosomal deletion marked **B** in the diagram is responsible for this condition. Which of the following genes, when hemizygously lost at this locus, is primarily responsible for the characteristic cardiovascular pathology observed in this patient?