Mass spectrometry analysis of CFH isolated from an AMD affected retina, using a novel monoclonal antibody that has restricted specificity to CFH nitrotyrosine residues, demonstrated that a number of CFH tyrosine residues that are nitrated in the AMD retina are contained in domains 4, 6 and 20 that serve important functional roles. The gene discussed is CFH; the disease is age-related macular degeneration.