American and European biologists that is meant to take the place of space programs as the current great consumer of public money in the interest of conquering nature. We know a great deal about what genes are made of and how they work at the most basic level. A gene is a long sequence of elements called nucleotides, of which there are only four kinds, identified by the letters A, T, C, and G. Every gene is a long string, of sometimes thousands or even tens of thousands of these A's, T's, C's, and G's, in a particular order: AATCCGGCATT and so on. This long sequence serves two functions. First, part of it specifies, like a code, exactly what the constitution of the protein molecules of our body will be. These proteins comprise the structural elements of which our bodies are made, the materials of our cells and tissues, and also the enzymes and hormones that make our metabolism possible. Corresponding to a particular sequence of A's, T's, C's, and G's, there will be produced by the machinery of the body a long molecule; a protein made up of simple elements, the amino acids. Each gene specifies the molecular makeup of a different protein. The particular sequence of amino acids that constitutes a particular protein is determined by the sequence of the nucleotides in the gene. If one or more nucleotides in the gene are changed, a different amino acid may be specified in the protein, which then may not be able to carry on its physiological function as well as before. In some cases, when a different nucleotide is substituted in a gene, less or even none at all of a particular protein may be manufactured because the machinery of the cell has a hard time recognizing the code. Second, other parts of the gene, also sequences of nucleotides, form part of the machinery that turns off and turns on the production of proteins. In this way, although the same genes are in every part of the body during every part of the life of an organism, proteins corresponding to some genes will be produced at some times and in some parts of the body whereas they will not be produced at other times and in other parts of the body. The turning off and on of the production of the body's constituents is itself sensitive to external conditions. For example, if the sugar lactose is provided to the coliform bacterium, the presence of the sugar will signal the bacterial machinery to start making a protein that will break down the lactose and use it as a source of energy. The signal to start translating the gene code into