Marked differences in phenotype in familial amyloid polyneuropathy (FAP) populations have been noted between but also within FAP populations. Initially, it was believed that patients with FAP, caused by the TTR V30M mutation, shared the same founder. However, recent studies have clearly shown that the V30M mutation in Sweden occurred spontaneously later in time than that in Portugal. The Swedish FAP-population's phenotype differs between various areas within northern Sweden. Thus the age at onset is in average 20 years earlier in Skelleftea than in Pitea areas, a distance of only 60 km. Age at onset appears also to have an impact on complications of the disease. Late-onset cases often develop a cardiomyopathy, especially male patients. Mitochondrial haplotype distribution has been noted to differ between early- and late- onset patients in the Swedish population. Mitochondrial function is one possible factor contributing to the differences seen both between and within populations.