Researchers are testing how drugs, and combinations of drugs, are affected in treating cancer, in a micro-gravity environment on the ISS. Larger cancer tumors, and tumors that are more spherical in shape, can be grown aboard the ISS. This gives researchers a more accurate picture of how cancer tumors form in the human body, and what is happening with the cancer tumors in the body. On Earth, the cancer cells being studied are not spherical. This research is just one example of what NASA is doing, in order to study how differently the human body reacts in a gravity free environment, which would be the case on a mission to a place like Mars. Researchers are also using rodents for stem cell research, and, they are also studying how "mold" behaves in a micro-gravity environment.