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The County of Baix Llobregat (Barcelona, Catalonia, Spain) presents a high prevalence of familial frontotemporal dementia (FTD) in the presence of P301L mutation in the MAPT gene. To evaluate a possible unique founder effect of P301L, and its age, the analysis of 20 single-nucleotide polymorphisms covering 50 kb and 12 single-nucleotide polymorphisms located along 30 Mb around the mutation was performed by developing 2 multiplex single-base extension reactions. In addition, families with affected and healthy individuals from France and Italy were analyzed. The FTD-affected individuals from Barcelona carried the same 50-kb haplotype linked to P301L mutation, suggesting a unique common ancestor, as opposed to French patients. Italian patients are also probably descendants of a unique ancestor, which would be different from that of Barcelona. Diversity of 30-Mb haplotypes found in Barcelona and the inference of the mutation age in these populations, among other reasons, suggest that prevalence of FTD linked to P301L MAPT mutation is the result of a locally originated mutation. • Two SBE reactions to analyze 50 kb and 30 Mb around P301L mutation were developed. • P301L mutation in MAPT in Barcelona is due to a unique mutational event. • This Barcelonan mutational event could be the result of a local mutation. • Haplogroup linked to P301L of Barcelona is also present in France. • Italy, likewise Barcelona, could have suffered another unique founder effect of P301L. [ABSTRACT FROM AUTHOR]