Furthermore, intellectual enrichment (as an environmental factor) was found to interact with genetics via a well-studied polymorphism (Val66Met) in the brain-derived neurotrophic factor (BDNF) gene.5 This link to BDNF is very interesting, as both the Huntington’s disease mutation and specific relevant environmental factors (including cognitive enrichment and physical activity) have been found to modulate BDNF expression in a transgenic mouse model of Huntington’s disease.7,8. The gene discussed is BDNF; the disease is Huntington disease.