Objective. To define the molecular basis of familial Mediterranean fever (FMF) in patients with only 1 mutation in the MEFV gene. Methods. Genetic analysis was performed in 20 FMF patients, including full sequencing of complementary DNA (cDNA) samples and multiplex ligationdependent probe amplification analysis. In patients with first-degree relatives with FMF, haplotype analysis was also performed. Results. A second mutation was found in 2 patients. In the other 18 patients, we could not identify additional mutations, large genomic deletions, or duplications. Analysis of single-nucleotide polymorphisms along the cDNA ruled out a lack of expression of 1 of the alleles. In 2 of the 3 families in which more than 1 sibling had FMF, we showed that the affected siblings inherited a different MEFV allele from the parent who did not have the MEFV mutation. Conclusion. These findings are highly consistent with the existence of a clinical phenotype among some patients who are heterozygous for FMF and could explain the vertical transmission in some families. A single mutation in the MEFV gene may be much more common than was previously thought and may include up to 25% of patients who are diagnosed as having FMF.