INS and diabetes mellitus: Similarly to humans, homozygous ablation of the Perk gene (Perk-/-) in mice recapitulates the defects of the human syndrome, demonstrating loss of insulin-secreting beta cells and development of diabetes, followed by the loss of glucagon-secreting alpha cells and failure of the exocrine pancreas, skeletal dysplasias followed by postnatal growth retardation (Harding et al., 2001; Zhang et al., 2002).