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Basma Haris,1 Idris Mohammed,1 Sara Al-Khawaga,2 Khalid Hussain1 1Department of Pediatric Endocrinology, Sidra Medicine, Education City, Doha, Qatar; 2Department of Dermatology, Hamad General Hospital, Doha, QatarCorrespondence: Khalid HussainDepartment of Pediatric Medicine, Sidra Medicine, Education City, OPC, C6-340 |PO Box 26999, Al Luqta Street, North Campus, Doha, Qatar, Tel +974-4003-7608, Email khussain@sidra.orgPurpose: To report a family with a homozygous INS promotor gene mutation causing permanent neonatal diabetes mellitus (PNDM) in one sibling and autoantibody negative childhood onset diabetes in another sibling.Case Presentation: Patient 1 is a 12-year-old girl born at term with low birth weight to a consanguineous family, diagnosed with PNDM at 26 days of life. She presented with ketoacidosis and has a severe course of disease with high insulin requirement. Patient 2 is a 9-year-old girl born at term with normal weight, who presented with ketoacidosis at 2 years of age. Both subjects have negative type 1 autoantibodies. On genetic testing, a mutation in the promoter region of INS gene c.-331 C>G was found in homozygous state in both subjects and in a heterozygous state in parents.Conclusion: Homozygous INS gene promotor mutations may present with either PNDM or later onset autoantibody negative diabetes in childhood. This suggests that homozygous INS gene promotor mutations show marked heterogeneity in clinical presentation within individuals in the same family. The pathophysiology of this is not well known but could be related to a number of factors, including the position of the variant, penetrance, other associated genetic defects, HLA etc. Premarital screening and genetic counselling is recommended for highly consanguineous families to reduce occurrence of such conditions.Keywords: pediatric diabetes, neonatal diabetes mellitus, type 1b diabetes, INS mutation