In 2017, Harvard University geneticist George Church predicted that gene-edited pig organs would be transplanted into people within two years—maybe just one.

“I was wrong,” Church now admits.

A startup he cofounded, eGenesis, had made news for its ambitious plans to use CRISPR gene-editing technology to modify pigs so their organs could be safely transplanted into humans without being rejected. That could solve a critical shortage of human organs available for transplant.

But no human test has yet been carried out. Instead, the company is currently testing organs from its pigs in monkeys at Massachusetts General Hospital in Boston. The experiments are being led by the hospital’s chief of transplant surgery, James Markmann.

“What we’re doing is a necessary step,” says Markmann, also an adviser to the company. “We’d be hard pressed to put a modified organ into a human until it’s been tested in a large animal.”

Neither Markmann nor eGenesis would detail which organs are being studied, or which species of monkey is involved in the experiments, which both say involve the most highly-engineered pig organs that surgeons ever created.

Doctors have dreamed for decades of using pigs to solve the organ shortage by transplanting their kidneys, hearts, and even lungs into human patients to replace organs that have stopped working. Right now, more than 100,000 Americans are waiting on transplant lists.

In the last few years scientists have achieved some major milestones toward such “xenotransplantation.” Researchers at the National Institutes of Health have kept pig hearts beating inside baboons (alongside the monkey’s own heart) for about two years, and German surgeons reported late last year that several baboons had survived for about six months after their hearts were swapped with one from a pig.

Those experiments were carried out using pigs genetically engineered by Revivicor, a subsidiary of United Therapeutics. The animals have genetic changes meant to prevent immediate human rejection of the organ, stop blood clots, and offset other types of immune attacks.

Thanks to such scientific advances, transplant surgeons are now debating how soon a test in a human might be risked. “We’ve got a Chevy. We may even have a BMW now. Do we wait for a Ferrari? There’s a point where you just want to give it a test drive,” says Devin Eckhoff, director of the medical school transplantation division at the University of Alabama at Birmingham.

Before pig organs can be tested in humans, however, there are still some key problems to overcome. Results with monkeys haven’t been very consistent, regulators haven’t said publicly under what conditions they’ll agree to a human test, and there’s debate over just how extensively modified the pigs should be.

Church’s company, eGenesis, leapt to notoriety by touting the role of CRISPR gene editing in swiftly making pigs with multiple genetic modifications—more than had previously been possible. In 2015, cofounder and chief scientist Luhan Yang demonstrated that she could carry out 62 simultaneous edits, used to deactivate viruses that naturally lurk in the pig’s genome.

In addition, Yang says her company has now introduced a “double digit” number of gene edits (both cutting and adding genes) to make the organs less likely to trigger immune rejection. These alterations are probably similar to those created by Revivicor. She has called her pigs the “most advanced” genetically modified animals on earth.