MASS1 (for monogenic audiogenic seizure susceptibility 1) is one of the largest known GPCRs and is therefore referred to as Very Large G protein-coupled receptor 1 (VLGR1) (1,2). MASS1 is a large, calcium-binding GPCR expressed in the central nervous system and the eye (2,3). MASS1 has a large ectodomain containing multiple calcium exchanger beta repeats that resemble regulatory domains of sodium-calcium exchanger proteins (3). The human MASS1 gene maps to chromosome 5q14 and encodes a 1967 amino acid protein (1,2,4). The MASS1 gene has been linked to the autosomal recessive inheritance of general epilepsy in Frings mice that have seizures in response to loud noises (5).