In labs across the globe, synthetic biologists are inching closer to a radical milestone: the creation of “mirror life,” a form of biology built from molecular structures that are the mirror image of those found in nature. While the science is dazzling, the ethical implications are sparking intense debate.

Mirror life flips the chirality of biological molecules—right-handed DNA instead of left, left-handed amino acids instead of right. These mirror-image molecules are harder for natural enzymes to break down, making them more stable and potentially useful in medicine. In fact, the FDA has already approved etelcalcetide, a drug made from mirror-image amino acids, to treat chronic kidney disease.

But what happens if scientists go further and build entire mirror-image cells?

John Glass of the J. Craig Venter Institute warns that such cells could be dangerously unrecognizable to natural biology. “Pretty much everybody agrees that would be a bad thing,” he says. Mirror cells might evade immune systems, resist decay, or even spread uncontrollably in the environment.

Some researchers, like Kate Adamala of the University of Minnesota, argue that the risks outweigh the rewards. “There is no benefit of mirror biology that couldn’t be achieved other ways with normal biology,” she says. Others, like Sven Klussmann of Aptarion Biotech, caution against premature restrictions that could stifle innovation.

The debate is no longer theoretical. Several labs are close to building synthetic cells using natural chirality, and the tools to flip those cells into mirror versions are emerging. In response, the Mirror Biology Dialogues Fund has convened global meetings to define ethical boundaries. In Paris, scientists urged funders to avoid supporting mirror-cell research. More discussions are underway in Manchester and at the U.S. National Academies.

As synthetic biology races ahead, the mirror life debate forces a reckoning: How far should science go in reimagining life itself?