Steve hadn't quite expected his child to be affected by the serum. Science hadn't had a chance to discover if the genes affected by the serum would be able to pass on to his children, and Kathryn's mother was, well, normal. So he figured his children would be safe.

Why was he always wrong about these things?

He had been playing with Kathryn, teasing her with a bear. He inched the bear into her reach, and when one of her hands stuck out to grab it he would quickly pull it away. But it was getting close to lunch time, so Steve let her catch it. Only, when she tried to pull it back, she just took the bear's head. And her giggling about it made it somehow ten times worse for Steve as he stared at the decapitated bear.

"How'd you do that?"

Kathryn just giggled at him, "Bear, Papa!"

"Yeah, I see that, babydoll."

He decided to give Bruce a call. They met Bruce in his lab at Avengers Tower an hour later after they'd eaten lunch.

"She tore the head off of a teddy bear," Steve said as he set Kathryn on one of the lab tables and helped her pull of her jacket.

"And you want me to...?" Bruce had an idea, but he had to be sure.

"Please test her for the serum."

Bruce nodded and sighed, "I thought you might end up asking at some point."

Steve distracted Kathryn while Bruce took a blood sample and began to analyze it. Steve entertained Kathryn for the next few hours around the Tower while he waited for Bruce to finish. He got a text sometime after 4:00 telling him to head back to the lab. Bruce showed Steve the codes that were Kathryn's DNA, and a previously collected code that was Steve's DNA.

"You see these genes?" Bruce asked, pointing to a couple sections.

"Mhm."

"Those are genes like her eye and hair color. She got those from her mom. But there are some here, like skin color," he pointed at two matching sequences from Steve and Kathryn's codes, "that match. Now, there are also genes that regulate the growth of cells and metabolism; that kind of thing. Looking at both of your genes, genes that she got from you, I can see that they have the same kinds of mutations."

"Does that mean...?"

"You have a second-hand serum baby."

Steve looked at Kathryn and she grinned up at him.

Steve sighed, "I'd hoped I wouldn't have to do that kind of baby-proofing."