During puberty in boys, aromatase converts some testosterone to estrogen, potentially causing breast bud development.

“Testosterone can be converted into estrogen by something called aromatase. I'll give an example of where this happens later in life, to just illustrate the principle and really embed it in your mind. During puberty in boys, XY chromosome individuals. It's not uncommon for there to be transient or sometimes long lasting breast bud development. Testosterone goes up during puberty, for the reasons we talked about before. And some of that testosterone gets converted into estrogen by an enzyme called aromatase.”