For example, the differentiation between asthma, COPD and asthma-COPD overlap encountered in firefighters who cleared debris from WTC relied on measuring eosinophilia and interleukin 4 (IL-4), which increased only in asthma-COPD overlap, and IL-21, which was higher only in isolated asthma and COPD [84]. This evidence concerns the gene IL4 and asthma.