FECH and autosomal erythropoietic protoporphyria: Nevertheless, among patients who carry the same mutation and the polymorphism there can be considerable variation in the severity of the phenotype, thus indicating that there are probably additional factors outside the FECH locus that impact phenotype in EPP, in particular transcription factors and modifier genes that might alter the level of expression of FECH. There are several other bone marrow genes in CBA/J mice that are differently regulated than those in C3H/HeJ mice (Tables 4 and 5).