It was a tiny gene, a microscopic difference on chromosome No. 15 that took Mike Ross’s firstborn in 2009.

Over time, he’s come to know, loathe and fight the syndrome that ultimately killed his 5-year-old, Tommy.

But how do you hit a neuro-genetic syndrome?

How can you take revenge on a tiny mutation called Angelman syndrome that took your son’s life?

Mike Ross, now 36, blogged about his boy, about losing a child and that first drive home, when he and his wife left their child’s body at the hospital.

“I don’t remember much of the drive home. I do know that my father drove us home,” he wrote. “Instead of having my son next to me, all I had was this stupid red book about grief.”

He wrote about what to say to a parent who has lost a child (not “at least you have other children”) and what it was like to look for a grave site for a child.

“Men need to know it’s okay to cry,” he told me.

He started obsessing about the research, about the doctors who are working toward a cure for Angelman’s at Vanderbilt University. He began a group called “Suck Feizures!” (transpose the first letters to get a feel for his anger), selling bumper stickers, organizing charity runs and wine-tasting benefits. He researched the life out of this confounding disorder, which hits one out of every 15,000 children.

It was one of Tommy’s countless seizures that killed him three years ago.

When he was born in the summer of 2004, “he was so pink and so chubby,” said Mike’s wife, Nani Ross, 41, as she showed me photo albums from that day at the hospital.

He was a gorgeous baby, just perfect. Even his Apgar score — the quick test hospitals do in the delivery room to assess the health of a newborn — was perfect.

But not long after they brought him home to Fairfax, he started losing weight.

“Failure to thrive,” Nani said.

Tommy wasn’t hitting his milestones like the other babies. There were tests and more tests. Then, the doctor told them to drop everything and come to his office.

Nani canceled a job interview. Mike left work, and they brought their sweet, smiling infant in.

“And we brought out a different baby,” Nani said.

The doctor told them Tommy had Angelman syndrome.

“Angelman’s syndrome?” they both asked.

Their child, they learned, would live a life of excessive laughter and seizures. He would have a stiff, jerky gait and no speech. The syndrome was originally described as “puppet children” because of the way they moved.

It was eventually named for the man who discovered it, Harry Angelman, and parents of children affected by it call them “angels.”

The syndrome results in children who are almost constantly smiling or laughing.

Nani, who works in banking, handled Tommy’s death differently from Mike. She was more private with her grief. When someone at her office learned that one of her children had died, she expressed surprise.

“She said, ‘But you don’t look like someone who lost a child,’ ” Nani told me. “And I wondered, what am I supposed to look like?”

She and Mike had another child, Alexander, who was 2 months old when Tommy died.

Tommy had his time as big brother, smiling and laughing at the baby, even petting him.

Just before his death, Tommy took his first steps in front of Alexander, showing him how it’s done.

“Oh, he’s going to work with special needs kids when he gets older, he has no choice,” Mike said. He and Alexander, now 3, water the bush they planted in Tommy’s memory nearly every day.

Mike has passed along most of the Suck Feizures work to other parents. “It’s doing well, the group. But I’m not as involved anymore,” he told me.

The anger stage is almost over for him.

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