Furthermore, a significant combined allele dose-effect was revealed such that the likelihood of MDD increased with the number of alleles.15 Revealing further genetic linkages, in a study by Yu YW et al it was observed that all patients of MDD who were homozygous for the -511T allele of the IL-1beta gene had less severity of depressive symptoms and more favorable Fluoxetine response compared to -511C carriers.16 This evidence concerns the gene IL1B and major depressive disorder.