Sir

Your News story “Geneticists prepare for deluge of mutant mice” (Nature 432, 541; 2004) promises mutants “covering every single gene”. But the section headed “It's a knockout” notes that many genes will be missed if a simple knockout strategy is exclusively used, owing to embryonic lethal knockouts and compensatory mechanisms.

Clearly, other strategies are needed if the promise to genome researchers is to be kept. The conditional knockout method proposed by a European consortium is intended for use in addition to the simple knockout strategy — not instead of it. Although the conditional knockout method may be more expensive and time-consuming than the simple knockout method, it offers a far more comprehensive picture of gene function, and allows the selection of somatic mutants, which are more closely related to human disease conditions.

It is only through the combination of methods that the News story's promise can become a reality.