Location of C3 mutations associated with aHUS. (A) Gene structure of C3, domains of the encoded protein, and position of the mutations. Note that the genomic structure numbering begins with the start site ATG, while the protein structures begin with the first amino acid of the mature protein. (B) Structures of complement component C3 and C3b showing the locations of mutations identified in atypical hemolytic uremic syndrome (aHUS) patients. Ribbon representation of C3b and C3 (2 views) with the domains containing mutations (labeled spheres).