PLK1 and small cell lung carcinoma: The decrease of miR-886-3p expression and hypermethylation of the miR-886 promoter and PLK1 are most likely potential biomarkers for adverse outcome prognosis and targets for SCLC therapy, due to the miR-886-3p-PLK1/TGF-b1 nexus regulation of SCLC aggressiveness.