The Making of Knockout Mice Knockout mice are the products of a genetic-engineering process that removes a particular gene. The result is a population of mice designed specifically for the detailed study of that gene's function. In essence, scientists can readily determine a gene's role if it has been inactivated. In the nucleus of every cell is a detailed instruction manual known as the genome. The precise sequences of nucleotides in DNA convey important information, spelling out the genetic code much like sequences of letters spell words.
Nucleotides are the rungs of the ladder of the DNA double helix.
Genes, in turn, are made of DNA, and chromosomes are made of genes. The process, called gene targeting, is a promising tool with which to study the human genome indirectly. An estimated 99 percent of mouse and human genes are the same. The sequence of nucleotides in a gene specifies which amino acids must be strung together to make a specific protein, the workhorses of cell biology. In the case of the research being reported today, the protein is an enzyme called nitric oxide synthase, which enables cells to make nitric oxide.