Another study examined the influence of genetic sequence variants (GSVs) in the vitamin D3 metabolism pathway, candidate-based GSVs, i.e., VDR, GC, CYP24A1, CYP27A1, CYP27B1, and CYP2R1, on overall survival (OS) and second primary cancer (SPC) in head and neck cancer patients [338]. This evidence concerns the gene GC and head and neck cancer.