Results on receptor knockouts in animal experiments and human gene polymorphisms of MT1, MT2, and melatonin biosynthetic enzymes indicate an association of melatonin deficiency and dysfunction with numerous diseases, as different as forms of cancer, metabolic syndrome, diabetes type 2, rheumatoid arthritis, and various mood disorders (summarized in [32]). Here, MTNR1B is linked to rheumatoid arthritis.