The third intron of human ferrochelatase (FECH) gene contains according to NCBI, a poly-C (11) and a poly-T (24) tracts which are located approximately 900 bp upstream from the known splice modulating SNP IVS3-48 c/t. Ferrochelatase catalyses the last step in heme biosynthesis and a deficiency of this enzyme results in the hereditary disorder of erythropoietic protopoprhyria (EPP). During the course of mutation analysis in the FECH gene among EPP patients, we observed variations in the length of the poly-C and poly-T tracts. To study these variations, we analyzed a total of 54 individuals of Swiss and Israeli origins. Among them, 37 were control subjects (23 individuals with the genotype t/t and 14 with the genotype c/t), 10 were unrelated EPP patients (genotype c/M) and 7 were unrelated asymptomatic mutation carriers (genotype t/M). The length of poly-C tract varied from 10 to 16, that of poly-T tract from 22 to 24 in the study cohort. Statistic analysis showed that the low-expressed FECH allele (IVS3-48c) is associated with poly-C12, C13 and C15 and poly-T22. In addition, the segregation of poly-C and poly-T tracts was studied in two Israeli EPP families. Instabilities, as seen by both insertion and deletion of one nucleotide between two generations, were observed only in the poly-T tract. The function of the poly-C and poly-T tracts are yet to be explored.