Because of the inability to breed Emi/Emi mice and in order to mimic the genotype of the vast majority of human EPP patients, we decided to cross Emi/wt mice with homozygous animals of the Fech mutant line m1Pas (C.Cg-Fechm1Pas/J) (Boulechfar et al., 1993; Tutois et al., 1991). The gene discussed is FECH; the disease is autosomal erythropoietic protoporphyria.