The deficiency in insulin in diabetes could be caused by the lack of insulin-producing pancreatic β-cells – either by autoimmune destruction of said cells in type 1 diabetes or defective development of such cells, or by a decline in functional pancreatic β-cell mass caused by cell stress followed by cell death (as in Type 2 diabetes), where loss of function manifests as aberrant production and/or release of the hormone insulin from the β-cells. This evidence concerns the gene INS and diabetes mellitus.