Complement factor D, also known as D factor, also known as lipase, is a serine protease that is necessary to activate the complement pathway. Factor D is a member of the trypsin family of peptidases, encoded by the CFD gene in humans. Factor D is expressed at high levels in various tissues, including monocytes/macrophages, muscles, sciatic nerves, endometrium, kidneys, and intestines, especially in adipocytes. In patients with renal failure, the concentration of circulating factor D increases. Similarly, in patients with Fanconi syndrome, the proximal tubular function of the kidneys is impaired, and the concentration of D-factor in urine is also at high levels.