In 1941, Beadle and Tatum proposed on the basis of their study of mutants of the fungus Neurospora sitophila that genes control specific biochemical reactions.  They suggested that the functioning of an organism depends on an integrated system of chemical reactions controlled in some manner by genes.  They further noted that “It is entirely tenable to suppose that these genes, which are themselves a part of the system, control or regulate specific reactions in the system either by acting directly as enzymes or by determining the specificity of enzymes.” This line of reasoning gave rise to the “one gene–one enzyme hypothesis”.  In a retrospective article, Beadle discussed the status of the one gene-one enzyme hypothesis 10 years after it was  proposed.  Beadle commented on the Cold Spring Harbor Symposium meeting of biologists in 1951.  He noted “I have the impression that the number whose faith in one gene-one enzyme remained steadfast could be counted on the fingers of one hand—with a couple of fingers left over.” However by the early 1960s the concept that the DNA base sequence of a gene specifies the amino acid sequence of a protein became well established on the basis of numerous experiments.  For example, an experiment by Crick, Brenner, Barnett and Watts-Tobin in 1961 demonstrated that each amino acid in a protein is encoded by a corresponding sequence of three bases in DNA (called a codon).  Soon after this, the specific codon assignments for each amino acid were determined (see Genetic code).