"It may have a large proportion of its brain made up of human cells, but none of its behavior is made up of human behavior."

Said Madeline Lancaster, a developmental neurobiologist, quoted in "These Mice Have Partly Human Brains/The animals may help reveal the causes of brain disease in people and point the way to treatments. But researchers already are calling for new ethical guidelines" (NYT gift link).

What counts as "human behavior"? It says: "The human cells were still immature, [the] team found, resembling the neurons inside the brain of a third-trimester fetus. But they still managed to connect with the mouse brain, becoming active as the mice went about their lives. The researchers filmed the movements of the mice, finding only subtle differences from ordinary mice."

You're observing a creature with the body of a mouse. What could it do that would make you say it's exhibiting human behavior (especially if you had an interest in producing mice that could be used in research? When would you say, no, this is human enough that ethics require us to abandon them as test mice?

Maybe put a little mirror in the cage and see if it gazes at itself philosophically or practices dance steps.