Serum proteins that can cause AF include transthyretin (TTR), apolipoprotein A-I, apolipoprotein A-II, lysozyme, fibrinogen, gelsolin, and cystatin C A form that is speculated to be familial is caused by the serum protein leukocyte chemotactic factor 2 (LECT2); however, a specific inherited gene mutation for this latter type has not been clearly demonstrated.