Type 2 diabetes is one of the biggest health problems globally. It is caused by insulin resistance and decreased β-cell mass and function due to β-cells apoptosis and dedifferentiation. A novel spliced isoforms of CD59 were discovered in human and mouse pancreatic islets. Those isoforms lack the GPI anchor and instead have unique C-terminal domains. These isoforms were named Isoforms Rescuing Insulin Secretion 1 and 2 (IRIS-1 and IRIS-2). IRIS-1 and 2 exist in the cytosol of the β-cells, where they colocalize with the insulin granules and mediate the insulin secretion via interaction with the SNARE proteins complex. Antibodies raised against those isoforms showed that islets isolated from T2D patients have a lower expression of both IRIS-1 and IRIS-2, as compared to islets isolated from healthy donors. This thesis demonstrated the electropositive path at the C-terminal domain of the IRIS-1, which is responsible for localizing this protein into the nucleus. IRIS-1 binds to histones within the nucleus and may act as a transcription factor. We also showed that IRIS-1 is required for maintaining β-cells identity and function, as cells overexpressing IRIS-1 display higher expression levels of β-cells maturation makers, such as Urocortin 3, Pdx 1, Nkx 6.1, Maf A, and Maf B.