Genetically modified mice either lack a specific gene or gene-pair (knock-out mice) or carry a piece of foreign DNA integrated into their own chromosomes (transgenic mice), and are used to deduce the functions of particular genes. Studies are beginning to show that an animal’s environmental conditions can completely change the results of genetic studies.
自从20年前面世以来被用得越来越多的基因重组鼠使这一问题变得更加严重。基因重组鼠不是缺少一种特定的基因或基因配对（基因剔除鼠），就是携带有一个被植入它们自己的染色体的外来DNA（转基因鼠），并且被用来推断特定基因的功能。已经有研究显示动物所生存的环境条件可以完全改变基因研究的结果。