For related articles and more information, please visit OCA’s Genetic Engineering page and our Millions Against Monsanto page.

If you want to know whether the bread you’re about to buy was sweetened with corn syrup, you can check the label. The same is true if you’re concerned about preservatives, caramel coloring or artificial flavoring. By law, all of these ingredients must be listed on food labels.

But not genetically modified organisms, or G.M.O.s. The Food and Drug Administration does not require clear identification and labeling of food products made with genetically engineered plants.

Most consumers want that to change. Some 93 percent of respondents to a New York Times survey in January 2013 said they wanted genetically modified ingredients identified, even though only about half said they would avoid G.M.O. products. More than 1.4 million people have signed the Center for Food Safety‘s petition urging the federal agency to require G.M.O. labeling. Last weekend, marches were held in dozens of cities to protest the introduction of genetically engineered products by Monsanto and other developers.

Vermont this month became the first state to require labeling of G.M.O. foods, and Connecticut and Maine have passed similar laws, though they are contingent on other states enacting legislation. Food producers and developers of genetically modified plants and seeds poured millions of dollars into advertising in 2012 to defeat a California initiative requiring G.M.O. labeling, and they are pushing a federal bill that would bar states from requiring labeling. They insist the ingredients are safe and say there is no need for labels.

“Labeling space is very limited, and mandatory labeling would create an unnecessary stigma,” said Claire Parker, spokeswoman for the Coalition for Safe Affordable Food, which represents businesses and organizations opposed to G.M.O. labeling. She and other industry representatives point to the F.D.A.’s determination in 1992 that there was no need for mandatory labeling of bioengineered foods because there were no “material” or “meaningful” differences between bioengineered and nonbioengineered foods.

Genetically engineered plants contain DNA from other animal or plant species that is intended to give them traits that are considered desirable by the manufacturers. One of the more recent innovations is an apple that does not turn brown after it is sliced. Another is a strawberry that withstands freezing.