Membrane cofactor protein (MCP; CD46) is a widely expressed transmembrane complement regulator. Like factor H it inhibits complement activation by regulating Ob deposition on targets. Factor H mutations occur in 10-20% of patients with hemolytic uremic syndrome (HUS). We hypothesized that MCP mutations could predispose to HUS, and we sequenced MCP coding exons in affected individuals from 30 families. MCP mutations were detected in affected individuals of three families: a deletion of two amino acids (D237/S238) in family 1 (heterozygous) and a substitution, S206P, in families 2 (heterozygous) and 3 (homozygous). We evaluated protein expression and function in peripheral blood mononuclear cells from these individuals. An individual with the D237/S238 deletion had reduced MCP levels and approximate to50% C3b binding compared with normal controls. Individuals with the S206P change expressed normal quantities of protein, but demonstrated approximate to50% reduction in C3b binding in heterozygotes and complete lack of C3b binding in homozygotes. MCP expression and function was evaluated in transfectants reproducing these mutations. The deletion mutant was retained intracellularly. S206P protein was expressed on the cell surface but had a reduced ability to prevent complement activation, consistent with its reduced C3b binding and cofactor activity. This study presents further evidence that complement dysregulation predisposes to development of thrombotic microangiopathy and that screening patients for such defects could provide informed treatment strategies.