FECH and autosomal erythropoietic protoporphyria: The purpose of this study was to examine whether the level of expression of the nonmutant FECH allele is also a determinant of phenotype in a mouse model of EPP, using mice in which an exon 10 deletion of FECH had been introduced into the genome of mouse embryonic stem cells by homologous recombination (Magness and Brenner, 1999; Magness et al. 2002).