When a certain gland becomes cancerous in humans, it produces high levels of a particular protein. A blood test can determine the level of this protein well before a cancer of the gland could be detected by other means. Some doctors recommend that aggressive anticancer treatment should be begun as early as possible for anyone who is tested and is found to have high levels of the protein. Which one of the following, if true, most seriously weakens the doctors' recommendation? ['The blood test for the protein has been in use for some time to monitor the condition of patients who have been diagnosed as having cancer of the gland.', 'Before the blood test became available, about one third of all cases of cancer of the gland were detected in early stages.', 'So far, no patients whose protein levels were found to be normal have subsequently developed cancer of the gland.', 'Enlargement of the gland, a common condition infrequently associated with cancer, results in high levels of the protein.']