According to research on SCLC, activation of CD44-MAPK-PI3K signal transduction results in increased expression of urokinase plasminogen activator (uPA)/uPA receptor (uPAR) and multi-drug resistance gene (MDR) 1, resulting in increased invasiveness and multi-drug resistance phenotype of lung cancer cells (Gutova et al., 2007). This evidence concerns the gene PLAU and lung cancer.