Assay for activators. In fetal liver cells from HS-26^{ΔNFE2(nonAKR)/+(AKR)} embryos, the activity of the wild-type allele of HS-26 can be directly compared with the mutated allele, under different trans-acting conditions. This assay can be used to determine the ability of a transcription factor to function through the intact AP1/NFE2 binding sites.