The source of the proteins and lipids in drusen is also not clear, with potential contributions by both the RPE and the choroid. Several trace elements are present in drusen, probably the most concentrated being zinc. The protein composition of drusen includes apolipoproteins and oxidized proteins, likely originating from blood, RPE, and photoreceptors. Drusen composition also includes members of the complement system. Zinc in drusen has been suggested to play a role in drusen formation by precipitating and inhibiting the elements of the complement cascade, especially complement factor H.