I have found mold deposits in my rental house! Can I break my lease?

I recently discovered some deposits of mold in my rental house. Yuck! There was a bunch of it behind my dresser and a lot in the corner of the bedroom closet, too. I've been able to clean up a lot of it, but it is fairly pervasive. It has stained the drywall and it's just plain disgusting! I've also had some wastewater back-wash problems in this same rental house (waste water that is supposed to be flowing away from the house some times flowes back into the house and spills into the basement. The landords are always prompt to fix it, but it happens every year). This is the last straw! Do you think I have grounds to break my lease?