NKX2-5 and teratocarcinoma: To support the results obtained in ESC cardiac differentiation, we used teratocarcinoma-derived P19 cells [52], and, to verify the proper differentiation of P19 cells into cardiomyocytes, we evaluated the gene expression of key markers of cardiac differentiation: Nkx 2.5, Gata-4, Apj, and Mlc-2.