On October 25, 2000, the U.S. Food and Drug Administration (FDA) requested technical assistance from the Centers for Disease Control and Prevention (CDC) in investigating adverse event reports (AERs) of human illnesses that were potentially associated with consumption of genetically modified corn products. Prior to these reports, a protein named Cry9c had been inserted into genetically modified StarLink? corn; it subsequently and inadvertently was introduced into the human food supply. CDC conducted an epidemiological investigation that included (1) reviewing the AERs, (2) administering questionnaires to all people who experienced adverse health effects and manifested signs and symptoms consistent with allergic reaction, (3) obtaining relevant medical records, and (4) collecting serum samples for temporary banking. The investigation concluded that 28 people had experienced apparent allergic reactions. These people had also reported eating corn products that may have contained Cry9c protein. With the endorsement of U.S. Environmental Protection Agency's Scientific Advisory Panel which convened on November 28, 2000, CDC recommended that the banked serum samples be evaluated to see if they contained evidence of an allergic response to the Cry9c protein.An FDA laboratory developed an enzyme-linked immunosorbent assay (ELISA) method to detect antibodies to the Cry9c protein. CDC sent coded serum samples to FDA for analysis, including serum samples from the affected people and historically banked serum samples collected before Cry9c entered the food supply. CDC also sent serum samples from people identified as being highly sensitive to a variety of allergens. The ELISA method found that none of the CDC-submitted samples reacted in a manner consistent with an allergic response to the Cry9c protein.These findings do not provide any evidence that the reactions that the affected people experienced were associated with hypersensitivity to the Cry9c protein. The difficulties of this investigation highlight the importance of evaluating the allergic potential of genetically modified foods before they become available for human consumption.