And so what that means is to have a high affinity between the substrate molecule and the protein kinase. The sequence of amino acids around the side, around the amino acid, which is about to be phosphorylated, has to be correct because if the sequence of amino acids on the substrate molecule is not correct, that active side will not be able to bind onto that substrate molecule. So we conclude that the specificity of protein kinases depends on the amino acid sequence that directly surrounds that target residue that is about to be phosphorylated. Now, to actually see what that means, let's discuss a specific example of a specific protein kinase that exists inside our body. And this is protein kinase A or simply PKA. So what exactly is the importance?