Interestingly, there is some evidence for reduced serum klotho levels in COPD that appears to be driven by smoking (Kureya et al., 2016; Patel et al., 2016), and serum but not muscle levels of klotho appear to be predictive of quadriceps muscle strength (Patel et al., 2016). Here, KL is linked to chronic obstructive pulmonary disease.