Aim. The objective of this study was to describe the clinical characteristics of two siblings and their father carrying a C95Y mutation in the insulin (INS) gene. Methods/Results. A Danish patient, his sister, and his father were identified to carry the C95Y mutation in the preproinsulin molecule causing permanent neonatal diabetes. All three were diagnosed before 29 weeks of age, were born at term with near-normal birth weight, and were negative for GAD, ICA, IA-2, and IAA autoantibodies. The daily insulin requirement the first six months after diagnosis was <0.5 U kg(-1) day(-1) for both children. The father, insulin treated for over 40 years, has bilateral preproliferative retinopathy. Conclusions. These three cases further confirm the essential features of diabetes caused by INS mutations with proteotoxic effect. We conclude that patients with similar features must be investigated for mutations of INS gene.