“We’ve learned that the top-down approach doesn’t work,” said Supervisory Special Agent Edward You, who said he coined the “Brewing Bad” term and had held workshops for biotech students and companies. “We want the people in the field to be the sentinels, to recognize when someone is trying to abuse or exploit their work and call the F.B.I.”

No scientific team has yet admitted having one strain capable of the entire sugar-to-morphine pathway, but several are trying, and the Stanford lab of Christina D. Smolke is a leader. She said she expected one to be published by next year.

No one in the field thought there should be no regulation, she said, but suggestions that home brewers would soon make heroin were “inflammatory” because fermenting manipulated yeasts “is a really special skill.” Implications of research like hers should be calmly discussed by experts, she said, and Dr. Oye’s commentary “was getting people to react in a very freaked-out way.”

Robert H. Carlson, the author of “Biology Is Technology,” said restrictions were doomed to fail just as Prohibition failed to stop the home brewing of alcohol.

“DNA synthesis is already a democratic, low-cost technology,” he said. “If you restrict access, you create a black market.”

What is considered one of the last important missing steps, a way to efficiently grow a morphine precursor, (S)-reticuline, in brewer’s yeast, Saccharomyces cerevisiae, was published in Nature Chemical Biology on Monday by scientists from the University of California, Berkeley, and Canada’s Concordia University.

The leader of the Berkeley team, John E. Dueber, said it was not trying to make morphine but 2,500 other alkaloids for which reticuline is a precursor, some of which might become antibiotics or cancer drugs.