Proteins are typically made up of twenty amino acids.  The amino acids are inked together through peptide bonds.   In addition to the amino acid chain, this chain often has side chain modifications that are important in the function of the protein.  There are hundreds of thousands of proteins that have been identified with hundreds of different functions.  We have only begun to scratch the surface of understanding protein chemistry.

There are millions of unique species in the world-- and more are discovered every day. Each of these living animals has its own unique set of proteins.   So how would we identify and characterize a protein?  Fortunately, because of evolution, there is usually some sort of analogy between different living things and their respective proteins.   For example, just as there are many analogous structures between a human and a horse (The respiratory system, the digestive system, and the nervous system.) a horses possess myoglobin and hemoglobin, just as we do. The idea of cross-specie homology can make the task of identifying proteins somewhat easier. If, for example, cows possess a certain kind of actin (a protein commonly found in muscle), it would be logical to look for it in horses or sheep. If it were found there, then it would be logical to look for it in mice, rats, and even humans. Once the sequence and structure are known for one specie, they can be predicted for other animals with a certain degree of accuracy. While the exact sequence of the amino acids within the proteins may not be exact, it can be assumed that they are somewhat similar when we take into account the functions of the proteins. But the inexact sequence between species can be taken advantage of since it can be used to identify one animal's protein from another.