Is it permissible to kill pet mice?

We know that it is permissible to kill wild rats and mice. but this question is for **pet** mice, that we got so we would look after them. We bought what we thought were **4 female mice** as pets. we set up a cage for them and kept them well fed. It turns out **one of them was a male**. Now we have so many mice that we cannot keep. Is it permissible to kill them?