说明：  The effectiveness of cancer radiotherapy is compromised by the small proportion of patients who sustain severe normal tissue damage after standard radiotherapy treatments. Predictive tests are required to identify these highly radiosensitive cases and screening for mutations in genes could be used as a predictive test. Mutations in the tumor suppressor gene P53 are associated with a wide range of different cancers and may have prognostic and therapeutic implications. Methods for rapid and sensitive detection of mutations in this gene are therefore required. Some gene tests are used to clarify a diagnosis and direct a physician toward appropriate treatments, while others allow families to avoid having children with devastating diseases or identify people at high risk for conditions that may be preventable. One type of DNA testing involves comparing the sequence of DNA bases in a patient s gene to a normal version of the gene.