NPY expression declines postnatally with β-cell maturation, and its re-emergence in animal models of β-cell dysfunction, for example, in β-cell-specific Neurod1 knockout mice (Gu et al., 2010), or during diabetes (Rodnoi et al., 2017), is thought to contribute to altered β-cell identity and impaired GSIS. Here, NEUROD1 is linked to diabetes mellitus.