Low-level mosaic pathogenic variants are recognised as an underappreciated cause of monogenic disease but are technically challenging to detect, particularly in organ-specific conditions when affected tissue is inaccessible.<h4>Methods</h4>We systematically investigated low-level mosaic variants in individuals with congenital hyperinsulinism (CHI: n = 1252) or neonatal diabetes (NDM: n = 312), two opposing pancreatic disorders of insulin secretion. The gene discussed is INS; the disease is neonatal diabetes mellitus.