The two primary direct methods for determining the primary structure of a protein are mass spectrometry and the Edman degradation. The amino acid sequence may also be infered from the DNA or mRNA sequence encoding the protein. A method involving the controlled hydrolysis of proteins. Edmun degradation is acid hydrolysis with N-terminus labeling. The amino-terminal residue is labeled, selectively cleaved, and identified, a process repeated until the sequence is determined. Other methods for protein sequencing involve the use of digestive enzymes selective for certain specific peptide bonds.