RRM2 and head and neck squamous cell carcinoma: The pathway enrichment analysis based on the target genes of these miRNAs revealed mostly signaling molecules that represent direct P53 effectors such as PTEN, JUN and E2F1 as well as cell cycle regulators such as RBL2, CCND2, RRM2 and E2F1. The E2F transcription factor network including genes such as E2F1, RRM2, RBL2, KAT2B represents a crucial target of the four selected miRNAs in both primary HNSCC cultures, which might be due to the fact that E2F1 impacts thymidilate synthase expression, which is a major target of 5-FU as already discussed [51].