Figure 1.

Strain-dependent variation of phenotype in mouse models due to genetic modifiers. To search for modifier effects, a mouse mutant (m; black) can be crossed to a wildtype mouse from a different inbred strain background (white). Modifiers can act as enhancers that exacerbate the phenotype or suppressors that abrogate the phenotype. Alternatively, an inbred strain that does not carry modifier variants will have no effect on the phenotype.