I have three mice but I only need to scarify two in my experiment. Can I keep the leftover one mouse in the cage with itself?
The other option is to put it in another cage with other mice. But this one is a knockout mouse, and can I mix it with WT mice (all of them are females, and I could mark the KO mice with ear mark)?
Since mice are social creatures, it is generally bad form to house them individually unless necessary. (Lofts, wheels, or other things can be used if individual housing is required.) Mixing with WT is not problematic unless the KO you're working with has some kind of deficiency that would put it at risk while being co-housed, or potentially confound a future experiment via exposure. Look to your facility's IACUC guidelines for further recommendations for housing options, or just ask one of the attending techs/veterinarians about your institution's policies.
Edit:
Mr Waddington added something below that I had overlooked. He is correct that makes cannot be co-housed because they are aggressive to males outside of their litter mates. If male litter mates have been housed separately for any amount of time, you can not rehouse them together.
Since mice are social creatures, it is generally bad form to house them individually unless necessary. (Lofts, wheels, or other things can be used if individual housing is required.) Mixing with WT is not problematic unless the KO you're working with has some kind of deficiency that would put it at risk while being co-housed, or potentially confound a future experiment via exposure. Look to your facility's IACUC guidelines for further recommendations for housing options, or just ask one of the attending techs/veterinarians about your institution's policies.
Edit:
Mr Waddington added something below that I had overlooked. He is correct that makes cannot be co-housed because they are aggressive to males outside of their litter mates. If male litter mates have been housed separately for any amount of time, you can not rehouse them together.
If it is a female mouse, then putting it with other female mice will be a good idea, as pointed out by Rebecca. However if it is male, your only option is to put it with a female (if you want them to have babies). You mustn't put a male mouse which has been on its own together with other male mice as their is a high likelihood they will fight and injure each other.
