Frequencies of XPC c.2815A>C, XPD c.934G>A, XPD c.2251A>C, XPF c.2505T>C and ERCC1 c.354C>T single nucleotide polymorphisms genotypes and haplotypes of head and neck squamous cell carcinoma patients stratified by response rate and toxicity to chemoradiotherapy. Here, ERCC4 is linked to head and neck squamous cell carcinoma.