PLK1 and head and neck squamous cell carcinoma: The French group confirmed that although both proton and x-ray radiation activated the VEGFC promoter in HNSCC cells, cells exposed to protons showed lower levels of gene and protein expression of VEGF-C relative to x-rays, and that irradiation with x-rays prompted a more aggressive tumor phenotype in vivo, with increased angiogenesis as well as overexpression of PLK1 (polo-like kinase 1, an inhibitor of apoptosis) or TRF2 (telomeric repeat binding factor 2), which has been linked with poor prognosis in patients with HNSCC [26, 27].