D and L refer to the confirmation, or orientation, of molecules that make up amino acids that form proteins. While amino acid confirmation is difficult to determine in a lab, biological systems such as the human body are able to easily differentiate these two forms and will only use amino acids in the L-conformation to form needed proteins. The amino acids found in almost all naturally occurring proteins, including milk protein, are entirely in the L confirmation. The D form of amino acids is only found in a few isolated instances which mainly consist of short peptide chains of bacterial cell walls and certain peptide antibiotics.