National Geographic : 1993 Jun
In search of new parasite-resistant breeds, a geneticist at the Interna tional Maize and Wheat Improvement Center in Mexico inspects a seg ment of corn DNA (above). This is an early step in mapping a breed's genetic code. Ultraviolet light reveals aflatoxin, a mold byproduct known to cause cancer in animals, fluorescing on dent corn and in test tubes (below). A 1988 U. S. outbreak tainted tens of millions of bushels.