Eosinophils appear to be the key effector cells in asthma, and there is a positive correlation between increased numbers and activation of eosinophils and the severity of asthma.1, 2, 3 Increased numbers of eosinophils have also been found in the bone marrow of patients with atopic asthma.4 Eosinophils develop in the bone marrow, exit to the bloodstream and enter lung tissue in response to pro-inflammatory mediators such as eotaxin.5, 6, 7. This evidence concerns the gene CCL11 and asthma.