A guanylate kinase is a phosphotransferase that produces ADP and GDP from the substrates ATP and GMP. SAPAP2, also known as DAP-2 (Disks large-associated protein 2) and PSD-95/SAP90-binding protein 2, is a 1054 amino acid protein that localizes to the postsynaptic membrane of neuronal cells of the brain and kidney. SAPAP2 likely acts as a signaling molecule which interacts with the human genes DLG1 and DLG4/PSD-95. The gene encoding SAPAP2, DLGAP2, maps to human chromosome 8. Consisting of nearly 146 million base pairs, chromosome 8 encodes over 800 genes and is associated with a variety of diseases and malignancies. Schizophrenia, bipolar disorder, Trisomy 8, Pfeiffer syndrome, congenital hypothyroidism, Waardenburg syndrome and some leukemias and lymphomas are thought to occur as a result of defects in specific genes that maps to chromosome 8.