Depending on a cell line and tumour microenvironment, IL-9 demonstrates anticancer behaviour: IL-9 inhibits cell growth and initiates apoptosis in melanoma tissues; the release of tumour necrosis factor-alpha (TNFα), IL-2 and interferon-gamma (INFγ) suggests antitumorigenic influence on lung cancer; by regulating T-cell function and killing tumour cells, IL-9 plays an antitumorigenic role in colon cancer [29,30,31]. Here, IL9 is linked to lung cancer.