GPR50, also known as MTR1L, is a nonglycosylated seven-transmembrane G protein-coupled receptor that is related to the melatonin receptors MT1 and MT2. GPR50 is expressed in the hippocampus, hypothalamus, and pituitary and forms 130 kDa homodimers. It heterodimerizes with either MT1 or MT2, resulting in inhibition of MT1 but not MT2 function. An alternately spliced isoform of GPR50 has a 4 aa deletion in the large C-terminal cytoplasmic domain. The presence of this deletion as well as various polymorphisms in GPR50 have been associated with elevated serum triglyceride and HDL levels. The deletion may also be associated with the development of bipolar disorder. Human GPR50 shares approximately 70% aa sequence identity with mouse and rat GPR50.