I have a mouse, that no matter what I do to keep his cage clean, he stinks up the place - why is that?

I could clean his cage every day and thinks would still get stunk up. I have a friend , who owns 3 rats and they creat the same problem. Is it just really bad body odor ? There has to be a solution to the bad smell. I have 6 guinea pigs and their combined forces isn't anything as bad as my 1 tiny mouse.