The authors genotyped 5 potentially functional SNPs-x-ray repair complementing defective repair in Chinese hamster cells 4 (XRCC4) reference SNP (rs) number rs6869366 (-1394 guanine to thymine [-1394G→T] change) and rs28360071 (intron 3, deletion/insertion), XRCC5 rs3835 (guanine to adenine [G→A] change at nucleotide 2408), XRCC6 rs2267437 (-1310 cytosine to guanine [C→G) change], and DNA ligase IV (LIG4) rs1805388 (threonine-to-isoleucine change at codon 9 [T9I])-and estimated their associations with severe radiation pneumonitis (RP) (grade ≥3) in 195 patients with NSCLC.