ABCB4 and progressive familial intrahepatic cholestasis type 1: PFIC1 or Byler disease [OMIM211600] [2] and PFIC2 or bile salt export pump (BSEP) disease [OMIM601847] [3] are associated with a low or normal serum gamma-glutamyl-transpeptidase (GGT) activity, whereas PFIC3 or multidrug resistance protein 3 (MDR3) disease [OMIM 171060] [4] is associated with a high serum GGT activity.