Using mouse parabiosis models (i.e., two mice surgically joined to each other, leading to a vascular anastomosis) Villeda et al. (2011) further proved that exposure of a young animal to the systemic milieu of an older mouse is sufficient to induce severe cognitive impairments and reduced neurogenesis after 2–5 weeks, and identify blood-borne eotaxin-1 as one of the responsible factors. This evidence concerns the gene CCL11 and Cognitive impairment.