Several lines of evidence have confirmed the importance of Nod2 mutations for disease susceptibility in Crohn's disease. For tracing Nod2 evolution, exons 4a, 4e, 8, and 12 mutations were screened in a collection of 1,064 DNA samples from 52 worldwide populations. The overall allele frequency was 7.5% for single nucleotide polymorphism (SNP)5, 0.2% for SNP8, 0.3% for SNP12, and 0.4% for SNP13. Nod2 mutations are mainly Caucasian alleles with strong distribution dissimilarity between single populations and major geographical regions. This regional diversity of Nod2 mutations within Europe points to the regional existence of selection pressure (possibly through dairy-associated bacterial infections within Neolithic cattle farming populations). The SNP5 gradient between Africa and the Middle East and its absence in Asian and Native American populations indicate that the evolution of this variant occurred in the Middle East. As mutations in exons 4e, 8, and 12 were only found in association with SNP5, this variant may have allowed selection pressure to arise.