More than 1,500 different enzymes have now been identified, and many have been isolated in pure form. Hundreds have been crystallized, and the amino acid sequences and three-dimensional structure of a significant number have been fully determined through the technique of X-ray crystallography. The knowledge gained has led to great progress in understanding the mechanisms of enzyme chemistry. Biochemists categorize enzymes into six main classes and a number of subclasses, depending upon the type of reaction involved. The 124-amino acid structure of ribonuclease was determined in 1967, and two years later the enzyme was synthesized independently at two laboratories in the United States.