PTEN and pachyonychia congenita: miR-21, which has been found to be upregulated in several cancers, could confer GEM resistance in PC by affecting expression of multiple target genes, including phosphatase and tensin homologue deleted on chromosome ten (PTEN)/RECK, programmed cell death 4 (PDCD4), Bcl-2, FasL, matrix metalloproteinase-2 (MMP-2), MMP-9 and VEGF (Park et al., 2009; Giovannetti et al., 2010; Bhatti et al., 2011; Dong et al., 2011; Wang et al., 2013).