SUMO1 and type 2 diabetes mellitus: SUMO1, 2, and 3 are widely distributed in human tissue, and have been extensively studied,[11] while the expression of SUMO4 is limited to the lymph nodes, spleen, and kidneys, and is associated with type 1 and type 2 diabetes.[12, 13] SUMO5 is restricted to some specific tissues, such as the peripheral blood leukocytes and testes.[14] SUMO2 has 97% similarity with SUMO3, but only 47% sequence identity with SUMO1.[15] These SUMO isoforms also differ in their ability to form chains, which leads to signal variation in the SUMO pathway.