Based on the quantitatively confirmed relationship between the proportion of insulin+ cells and glucagon+/insulin+ cells in the islets, and on the presence of transient forms between insulin-containing and insulin-deficient islets in the pancreas of children with recent-onset T1D, it can be assumed that insulin-deficient islets are formed from the insulin-containing islets as a result of the loss of insulin+ cells and an increase in the proportion of glucagon+/insulin+ cells. Here, GCG is linked to type 1 diabetes mellitus.