This study provides the most comprehensive review to date of ethnic variation in the 5-HTTLPR polymorphism. Although variation in the frequencies of the S and L alleles were identified in earlier work, some studies have erroneously assumed that the 5-HTTLPR polymorphism is bi-allelic. Our review demonstrates that extra-long alleles occur in several samples of persons of African origin and East Asians, but they have not been reported in any Caucasian group.
