We have two males and one female. They're all litter mates. They're all fixed too. Our two males have been in fights with some of the strays in the neighborhood. They started marking everything in the yard and then one came in and marked one of my guitars. I moved all my instruments into the spare bedroom and closed the door. No cats allowed in there. Unfortunately for me, there's no real solution. The good thing is that both males go outside. It was only one of the two who marks inside. If you don't already have one, I would get a black light or UV type of flashlight so you can see where all the markings are. We did that and cleaned them up. It seemed to tamp down the behavior. I would get your cat fixed as soon as possible. Also, how many litter boxes are you running? If it's only one, you should add more. I heard that the rule is you have to have one litter box per cat, plus one extra. So, two cats = 3 litter boxes. Don't give up! Good luck! 
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