The concept is all mice in the USA come from one laboratory and have a common attribute that's different from wild mice and European suppliers.
And it's spelt out how in theory this different attribute could change medical results.
To come back with we don't care, without proof it's not a issue, is a deeply broken system, but is exactly how current universities work.
Here's the actual segment (18 mins) - https://www.youtube.com/watch?v=ve4q-1D_Ajo
Related (12 mins) - https://www.youtube.com/watch?v=8ygLNOt43So