High activity of aldehyde dehydrogenase (ALDH) has been found in stem and progenitor cells. High activity and/or overexpression of ALDH can be used as a marker of CSC in various types of cancers, including lung cancer. Overexpression of ALDH1 is associated with a poor prognosis in patients with lung cancer and more severe histological grade and stage of the disease (Jones et al., 1995; Ginestier et al., 2007; Huang et al., 2009; Jiang et al., 2009; Okudela et al., 2013; MacDonagh et al., 2017). The gene discussed is ALDH1A1; the disease is lung cancer.