Human membrane cofactor protein (MCP), also designated CD46, is a widely distributed complement regulator. In mouse, expression of MCP is largely restricted to testis, while a related widely expressed protein (Crry) appears to perform the regulatory activity of MCP. Crry (for murine complement receptor-related gene Y), protects the body from damage inflicted by inflammation and modulates renal interstitial disease induced by proteinuria. A functional ortholog of human decay-accelerating factor (DAF, CD55) and membrane cofactor protein (MCP, CD46), Crry protects cells from homologous complement by inhibiting the complement proteins C3 and C4 convertases from marking foreign cells for destruction. Sequence analysis suggests that Crry may be the mouse genetic homolog of the CD35 antigen encoded by the human gene CR1. Manipulation of Crry may play a role in treating central nervous conditions such as traumatic brain injury or stroke. Deletion of the gene encoding Crry may play a role in miscarriages.