Discloses a polypeptide comprising a complement factor B protein analogue, wherein the complement factor B analogue comprises a mutation of a free cysteine amino acid and is at least 90% identical to amino acids 26-764 of SEQ ID NOs: 1, 2 or 3； to amino acids 26-990 of SEQ ID NOs:22 or 23； to amino acids 26-480 of SEQ ID NO:2； or to amino acids 26-1003 of SEQ ID NO:26, wherein the sequences are as defined in the complete specification, and related methods, nucleic acids and vectors. These are used to modulate a complement pathway or for the study and/or treatment of various conditions or diseases related to a complement pathway.
