UDCA is a hydrophilic bile acid, nontoxic to hepatocytes,that is able to replace cell-damaging hydrophobic bile acids as wellas to stimulate the expression of MDR3 (in PFIC3) and BSEP (in PFIC2)by increasing phospholipid secretion.2 However,it has been reported that for many children with a complete absenceof MDR3 function, the efficiency of this treatment is limited anddoes not prevent the progression of a deleterious liver disease. The gene discussed is ABCB4; the disease is liver disorder.