SamOH

If you want to use/study the growth of tumors of human origin, then you need to use immunocompromised mice.  The reason being that such mice cannot immunologically reject the foreign (i.e. non-mouse) tissue.  The disadvantage of such a model is that you are investigating tumor growth (or perhaps an agent that can suppress tumor growth) in an animal that does not have a competent immune system.  To overcome this disadvantage, you could use a syngeneic tumor model.  In this kind of model, cancer cells or tumor fragments of mouse origin are injected/implanted into a mouse.  The murine tumor fragment or cancer cells should come from a mouse of the same strain (i.e. syngeneic) as that into which you are introducing the cancer cells/tissues.  This will prevent immunologic rejection based on strain differences.  Thus, the advantage of this syngeneic model is that you can study the tumor in the context of an intact immune system.  Also, immunocompetent mice are often less expensive than immunocompromised mice.  The disadvantage is that it is a mouse tumor, not a human tumor.

Hope that helps.