<b>Background:</b> The molybdenum cofactor (Moco) deficiency in humans results in the inactivity of molybdenum-dependent enzymes and is caused by pathogenic variants in <i>MOCS1</i> (<i>Molybdenum cofactor synthesis 1</i>), <i>MOCS2</i> (<i>Molybdenum cofactor synthesis 2</i>), and <i>GPHN</i> (<i>Gephyrin</i>). Here, MOCS1 is linked to hyperinsulinemic hypoglycemia, familial, 4.