Hemophilia would be “a lifelong thing,” said Jay Konduros. Then the brothers joined Spark’s gene therapy trial for hemophilia B.

The actual infusion of the experimental drug was anticlimactic, Jay Konduros recalled. He walked into a hospital in Philadelphia, sat in a chair and had an intravenous drip for half an hour. That was it.

Now levels of factor IX in Jay Konduros’s blood are around 50 percent. Bill, who also joined the trial, has levels closer to 75 percent. Neither has required any factor IX since their gene therapy.

Both struggle to accept the fact that, for the moment, their lives are very different.

“When I hit myself or strain a muscle or twist, I immediately revert to thinking like a hemophiliac,” Bill Konduros said. “You go on high alert. Is the ache spreading? Is it throbbing?”

One day in May, Jay fell, landing on his forearms. Both wrists hit hard on concrete, and he struck the left side of his thigh, already damaged from previous bleeds.

He took a few deep breaths and told himself, “You will be O.K., you will be O.K.”

He worried, anticipating disaster. That night he stretched. He examined himself. Nothing seemed damaged. He woke up in wee hours of the morning and nervously examined himself again.

He was fine. He waited three days to call his brother and tell him: He was now a normal person who had a minor fall.

“You hear a lot of things described as miracles or miraculous,” Bill said. “I guess I would say this truly is.”