Aspartame is an artificial sweetener that's about 200 times as potent as sugar. Although it's technically an artificial substance and doesn't occur in nature, aspartame is the result of a combination of two natural amino acids. Two different species of bacteria produce these acids, and in some cases, one of the bacteria is modified in order to boost the yield.
So is aspartame dangerous? If it is, it's not because of genetic manipulation -- aspartame itself contains no genetic material. However, debate still rages in the food world about how safe it is for human consumption. While there's no confirmed link to cancer in humans, female lab rats fed aspartame had higher rates of lymphoma and leukemia than those in the control group.