Mapping of the tested FH variants. FH N-terminal (A) and C-terminal (B) crystal structure. The surfaces that interact with C3b (N terminus) and C3d (C terminus) are depicted in yellow. The FI-interacting surface in the N terminus and the sialic acid binding site in the C terminus are depicted in purple. The position of all variants is indicated in orange. FH variant classification codes are based on functional assay results: benign (green), nonexpressed (black), and functionally impaired (red).