The coinheritance of a low-expressed FECH allelic variant and a mutated FECH allele lead to phenotypic expression of the disease in 5 unrelated EPP families. Solid arrows indicate the probands. Lymphocyte FECH enzyme activity (in parentheses) is expressed as nanomoles of Zn-mesoporphyrin per hour per milligram of protein at 37°C (control value, 5 ± 1.5; mean ± 2 SD). Specific mutations identified in 1 FECH allele are listed in the left box. Haplotypes were constructed from 5 polymorphic loci as listed in the right box. Haplotypes that are associated with the low-expressed alleles are in red, the normally expressed ones are in black (area in white), and the mutated ones in blue. The relative amount of each allelic FECH mRNA was determined using a fluorescent primer extension assay. The generated peaks (one of them is indicated for each normal parent) and their relative areas (R; see Subjects, Materials, and Methods) are indicated in square brackets. Data are means resulting from at least three independent experiments. Alleles that segregate from the normal parents (I₁2, I₂1, I₃2, I₄2, and II₅2) to the probands (II₁2, II₂1, II₃1, II₄1, II₅1, and III₅2) carried a common 5′ partial [G-T-A9] haplotype and were associated with a lower steady-state mRNA level. This decrease is indicated by the relative areas (R) between allelic FECH mRNAs ranging from 1:0.5 (I₁2) to 1:0.74 (I₄2). In family 5, haplotyping showed that the mother II₅2 had transmitted the low-expressed allele to her three children. Furthermore, the daughter (III₅3), a clinically normal subject, had inherited the normal allele from her affected father and had a 50% decreased FECH enzyme activity, in agreement with the coinheritance of two low-expressed alleles.