Autoimmune insulitis in patients with type 1 diabetes mellitus (T1DM) is characterized by deposits of hyaluronan (an extracellular matrix polysaccharide) in the islets of Langerhans. However, it was unclear what role, if any, these deposits had in the disease. New research has demonstrated the hyaluronan deposits are associated (both temporally and anatomically) with the development of insulitis in the DO11.10xRIPmOVA mouse model of T1DM. Treating these mice with 4-methylumbelliferone (which inhibits hyaluronan synthesis) prevented the mice from developing diabetes mellitus. The treatment prevented the accumulation of hyaluronan and increased the numbers of FOXP3+ T regulatory cells. The researchers suggest that as 4-methylumbelliferone is already approved for treating biliary spasm, it could be repurposed to prevent the development of T1DM in at-risk individuals.