The entire precious payload for each patient would be contained in 150 microliters of saline solution - less than one-thirtieth of a teaspoon.

The surgery

Al Maguire decided two days in advance just where he would insert the needle into Tommaso's right eye.

In October, with Tommaso's sister Josalinda, he'd injected the virus away from the center of the retina, in case it somehow impaired what little vision she had.

There were no complications with Josalinda, so Tommaso would get his injection closer to the center.

Maguire had been thinking intently about the surgery for days.

He had done hundreds of the operations on baby mice, whose eyes are the size of a pencil tip, and he'd done dozens of dogs.

The night before, to relax, he'd watched The Pianist, the tragic movie about a Jewish musician who lives in Poland during the Holocaust. Maguire found the context helpful; if others could endure such pain, surely his own challenges were insignificant.

The day of the surgery, the hospital thawed out the vial containing the virus. Research coordinator Kathy Marshall carried it down the hall in a cooler.

Maguire's surgical team prepped Tommaso, marking the red spot on his forehead. He was draped and put under anesthesia.

Because Tommaso was young, the clear substance inside his eye - called the vitreous - was still fairly firm and jelly-like. The needle, a flexible instrument called a cannula, could not be inserted all the way back to the retina.

So first, the surgical team removed the jelly and simultaneously replaced it with a saline solution - a procedure called a vitrectomy.

The surgeons cut three tiny holes in the surface of the eye: one for a vitrector, an instrument to snip and gently suck out the jelly; one for an infusion port to pump in the saline solution; and one for a "light pipe" to illuminate the back of the eye.

Maguire watched through a microscope as he and his team did the work. He operated in stocking feet because he had to control two of his tools, the vitrector and the microscope, with foot pedals. Each had a series of switches that he operated with his toes.

Outside, in the waiting room, Maguire's wife - the scientist - was nervous.

"It's scary, embarking on unknown territory," Bennett said. "At a certain point, it's time to say it's safe from every aspect we've looked at in animals. But humans is a different game."

It was time for the injection.

Maguire held the light pipe in his left hand. He carefully inserted the cannula with his right.

The flexible needle went through the sclera - the "white" of the eye - then through the saline fluid, until Maguire felt it come to rest at the back of the eye, gently, against the retina.

The needle was connected to a syringe in the hands of Penn ophthalmologist Eric Pierce, who stood ready to push the button that would release the precious genetic material.