A method of treating a disease associated with a pathological cell population is disclosed. The method comprises increasing an association of complement with the pathological cell population in a subject in need of treatment, and decreasing a level and/or activity of a polypeptide in the pathological cell population, the polypeptide being at least 70 percent similar to SEQ ID NO: 1. A method of treating a disease associated with a pathological cell population is further disclosed. The method comprises administering to a subject in need of treatment a substantially cell membrane-impermeable compound for decreasing a level and/or activity of a polypeptide in the pathological cell population, the polypeptide being at least 70 percent similar to SEQ ID NO: 1; and, optionally, increasing an association of complement with the pathological cell population. A method of treating a disease associated with pathological complement-mediated cytotoxicity is yet further disclosed. The method comprises increasing in a cell population of a subject in need of treatment, a level and/or activity of a polypeptide at least 70 percent similar to SEQ ID NO: 1.