There are two specific gene variations that have been linked to a pharmacogenetic profile that predicts the extent of tumor regression (TRG). These variants are known as hOGG1-1245C>G, which may affect radiation sensitivity, and MTHFR-677C>T, which plays a role in fluoropyrimidine function. Patients who have one of these genetic variants are less likely to achieve a TRG ≤ 2. It is important to consider these genetic factors when evaluating the potential effectiveness of a treatment plan. The gene discussed is MTHFR; the disease is neoplasm.