Kay Higgins, his lab manager, can't define his humor either, but knows it when she sees it.

It comes out with no warning. He simply launches into rare but funny stories about things that happened. Then he disappears back into his office. Around Christmastime, he came out of his office carrying a beautifully wrapped box he'd received. Open it, he urged.

Inside were mice finger puppets, their dance controlled by his fingers through holes in the box's bottom. "I started laughing and he started laughing. Now it's in his office on a shelf. I don't think he ever showed anyone else. He has a good sense of humor that's not predictable. I think I happened to be nearby when he wanted to share it with someone, really."

Fraser doesn't have trouble explaining other Capecchi traits. "He is very kind," she says. "He is very respectful of other people and gives people a lot of room.

"I think he doesn't communicate his ideas or his personality to other people very easily. He does that better in science than in the personal arena.

"He's a generous person. But trusting is a different word. I think he more easily trusts women than men."

It is from her husband, she says, that she has learned patience. "I think his patience has been very helpful for him. The other thing — sometimes a bad quality and sometimes a good quality — is he's incredibly tenacious. He will not give up."

A day in the office

Capecchi starts each day with a run. When he was younger, he ran marathons.

When Fraser and Capecchi met, he was "in the lab all the time," she says. These days, administrative work takes up much more of his time. Even before that, he was already doing more writing than bench work. His roster of published articles is impressive.

Fraser thinks he misses the lab work, but "he doesn't make changes to go back," she says, "so I'm not sure."

Higgins and Linda Oswald, his administrative assistant for 12 years, share gatekeeper duties. Is now an OK time to talk to Mario? colleagues ask. It's not that he's moody. "The word that comes to mind is 'focused,"' Oswald says. "You'll get his full attention when he's through with what he's working on.

"He gets going on something and everything else gets totally ignored until that particular project is finished."

Despite his focus, he's not particularly organized. "He knows where everything is, though it looks like a cyclone hit."

He's demanding but fair, they say.