Estimate of agouti expression levels in the livers of alb-agouti 86, alb-agouti 83, BAPa20, lethal yellow ( Ay/ a ) and viable yellow ( Avy/ a ) mice. A Northern blot of poly (A)+ RNA, isolated from the liver of a single mouse for each of the genotypes indicated, was hybridized with a radiolabeled agouti cDNA probe, then stripped and rehybridized with a radiolabeled Gapd cDNA probe to control for the amount of mRNA loaded for each sample. Levels of agouti mRNA relative to Gapd mRNA levels were quantified for each sample by densitometric analysis (see Methods). The level of agouti mRNA expression in lethal yellow mice was assigned the value of 1.0, and the expression levels in viable yellow mice and transgenic mice were normalized relative to this value. The viable yellow mice used in this assay had coat colors that were completely yellow (y), mottled yellow and agouti (m), or pseudoagouti (p).