The unambiguous identification of lung cells that express this protein remains difficult and controversial because the up-regulation of ABCG2 is mainly drug-induced, resulting in a multidrug-resistant cell phenotype Therefore, in some types of cancer, e.g., myeloid leukemia [51] or lung cancer [53], a high expression of the ABCG2 is associated with a negative patient prognosis. Here, ABCG2 is linked to cancer.