News Release

In the nucleus of every cell is a detailed instruction manual known as the genome. It is written in the form of nucleotides, four of which constitute the entire alphabet -- adenine (A), cytosine (C), guanine (G), and thymine (T). 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.

By changing the nucleotide sequence of a chosen gene, scientists can inactivate that gene, and it is said to be "knocked out" of the genome. 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.

But making knockout mice first requires a painstaking laboratory process in which a gene is "marked" precisely where the instructions for the desired amino-acid sequence are located. The marked gene is placed into mouse embryo cells, which are treated with drugs to produce a mouse genome lacking the specified gene in one chromosome.

Then the mutated embryonic cells are inserted into young embryos and grown in surrogate mothers. But not all of surrogate offspring will be knockouts. In some, the cells carrying the mutation do not survive. Scientists must identify the knockout males and females and breed them, producing offspring containing only the altered chromosome.

Embargoed for release at 4 p.m. est Nov. 22, 1995