Omega-3 fish oil supplements may fail without the presence of the key enzyme ALOX15. The researchers found that the omega-3 fatty acids EPA and DHA suppress colorectal cancer only when the ALOX15 enzyme is present. Without this enzyme, fish oil sometimes increased the growth of tumors in mice, especially DHA. These results show that not all omega-3 supplements work equally well. Genetics can determine who really benefits from supplements. Fish oil's cancer-fighting reputation thus depends on this little-known gene.[1]