Complex sentence: A genetic predisposition to disease has been thought important for some time, as evident by cases of PBC in family members, concordance in identical twins, and clustering of autoimmune diseases.
Split into simpler sentences: A genetic predisposition to disease has been thought to be important for some time. Evidence for this includes cases of PBC in family members, identical twins both having the condition (concordance), and clustering of PBC with other autoimmune diseases.