CXCL12 and chronic obstructive pulmonary disease: While the observations of concordant, numerically reduced mean levels of serum CXCL12 in the smoker as opposed to non-smoker subgroups of IPF subjects (Fig. 5D) and healthy subjects (Fig. 5E) were not statistically significant, by pooling all non-smokers (healthy and IPF) compared to all pooled smokers (healthy, IPF, and COPD), there was evidence to suggest a lowering of CXCL12 levels in the current smokers (U = 104, p = 0.033, Fig. 5F).