They were not "making the mice transgender".

From the Whitehouse's 6 evil studies list:

Three of them were studying the long-term effects of gender-affirming hormone therapy on the growth of the skeleton, how it affects fertility, and if it changes your breast cancer risk. These are all things that people who are against transgender care use as reasons to stop transgender care. So they were potentially a way for the government to say "no" to this kind of hormone therapy and have some scientific backing for why. (Note: the results of these studies weren't going the way anti-trans numpties would have liked. I'm sure that has nothing to do with it though.)

One is studying (as best I can determine) how GA therapy works, I guess because we don't really fully understand it. This one used "transgenic" mice (scary!) to help isolate the processes. That means they were genetically modified.

There was a very scary one about how such hormone therapy affects the effectiveness of the HIV vaccine. (They made sure to list this one first with its homophobantitransantivax trifecta).

One study they claimed in their list was actually just a comparison of asthma between men and women (nothing about transgender, but it mentioned "gonadal hormones" which is like, so gross).
Edit to add: This one also accounts for over 1/3 of their claimed savings and is only tangentially related to transgender women because of the estrogen they take. It was mainly focused on laying a basis for future therapies for everyone with lung disease.

A bunch of the info came from here: https://www.motherjones.com/politics/2025/03/white-house-scared-of-trans-mouse/

I also read through the abstracts from the studies listed and linked in the Whitehouse press release.