So let's suppose we have a sequence of 200 amino acids and let's say 20 of them are actually serene or three Anion amino acids. The question is how does protein kinase A actually know which Serine or three an amino acid to actually bind to? What determines the specificity of that protein kinase A? So it's exactly what we said before. It's the sequence of amino acids that is found around the side that residue that is about to be phosphorylated that determines the ability of that protein kinase Apka to actually bind onto that substrate molecule. Now, what is the sequence?