GCG and type 2 diabetes mellitus: Aside from dedifferentiation, lentiviral-mediated lineage tracing and observation of islets from individuals with T2D discovered a process of transdifferentiation, in which β-cells can be converted into other islet cell types, such as glucagon-producing α-cells (Spijker et al., 2013; Cinti et al., 2016) and somatostatin-producing δ-cells (Cinti et al., 2016).