As a prosecutor in the Brooklyn DA’s office for nine years, Kieran Holohan used the DNA of countless hoods, crooks and nogoodniks to win cases.

Now he needs yours.

The ex-prosecutor, now a defense lawyer, was diagnosed with leukemia Dec. 9 and faces grim odds if he can’t find a bone marrow donor.

“It’s a gripping fear,” he said of the diagnosis, delivered a mere eight months after becoming a father, and just a day after he was honored by his amateur rugby club, the Village Lions. “You’re basically being told that you’re going to die. My heart started racing. Shortness of breath. That lasted maybe 25 to 230 ssecond, and then i was like, what do we do?”

Supporters from the rugby community and both sides of the criminal justice system have teamed up to help find a bone marrow match for Holohan. Potential donors can have their cheeks scraped for a DNA sample, and can also donate money to cover the costs of testing, Sat., Jan. 23, at Turtle Bay’s Cornerstone Tavern at 4 p.m.

Holohan said he’s hoping for a turnout of 300 to 500 people. About 275 have already confirmed on a Facebook page he set up for the event.

“The reality is this,” he said. “I recognize that I may not find a match out of the 300 to 500 people, but there are 300 to 500 people who will be tested, and may save the lives of other people.”

The odds are not good.

Siblings provide the best chance of a match, somewhere between 1 in 3 and 1 in 4. Unrelated people have a 1 in 20,000 chance of matching up.

Holohan said his siblings have already been tested, but it’s too soon to know the results.

He said without a bone marrow transplant – relying just on chemotherapy – he has about a 30 percent chance of survival. With a transplant, those odds jump to 80 percent.

“I want to be at my daughter’s wedding,” he said. “It’s all I think of. This might kill me. It might not. I’m not afraid of the death. I’m afraid of my daughter not having a father.”

Ironically, as hundreds of his friends and supporters gather on Saturday to offer their DNA, hundreds more he helped put away are in prison, their DNA on file in a database.

“You’d have to ask for that person’s permission,” he quipped. “I’m not sure I’m getting that.”