Genes are now known to be implemented as sequence of genetic code that direct specific cells to produce a particular protein at a particular time. An essentially infinite number of possible different protein molecules can be produced depending on the particular order of amino acid molecules used in their construction. The code for protein production has been "broken" so that we now know that a three letter sequence (a codon) is used to specify a particular amino acid (there are 20 amino acids). For instance the sequence GGC specific that the amino acid glycine is to be added to a protein molecule. Start and stop codon mark the beginning and end of protein coding sequence in a manner startingly like modern data communications schemes. There are 64 possible codons and only 20 possible amino acids. So some redundancy and error correction exist. The regulatory code sequences in genes that specify in which parts of the body and/or at what times a protein will be produced are more complex and less understood.