Gilbert syndrome is caused by a modified gene you inherit from your parents. This gene usually controls an enzyme that helps break down bilirubin in your liver. When you have an ineffective gene, your blood contains excess amounts of bilirubin because your body doesn't produce enough of the enzyme.
How the body processes bilirubin
Bilirubin is a yellowish pigment made when your body breaks down old red blood cells. Bilirubin travels through your bloodstream to the liver, where the enzyme breaks down the pigment and removes it from the bloodstream.
The bilirubin passes from the liver into the intestines with bile. It's then excreted in stool. A small amount of bilirubin remains in the blood.
How the modified gene is passed through families
The modified gene that causes Gilbert syndrome is common. Many people carry one copy of this gene. In most cases, two modified copies are needed to cause Gilbert syndrome.