Alternatively, there may be individual differences in the expression of CD44[39], making smokers with low constitutive CD44 expression more susceptible to the development of COPD since, as discussed above, CD44-deficient mice succumb to unremitting inflammation after lung injury with impaired clearance of apoptotic neutrophils and impaired activation of TGFβ[31]. Here, CD44 is linked to chronic obstructive pulmonary disease.