The monoclonal antibody 4H2A9 reacts with the CCP1 domain, which is shared by MASP-1, MASP-3 and MAp-44. In total 5MASP proteins have been identified (MASP-1, MASP-2, MASP-3, MAp19 and MAp44) which are encoded by 2 genes. The MASP-2 gene gives rise to MASP2 and MAp19 due to alternative splicing, whereas the MASP-1 gene encodes a primary transcript which can be spliced to three different mRNAs coding for MASP-1, MASP-3 and MAp44 having molecular weights of 88, 89 and 44kDa, respectively. On the protein level MASP-1 and MASP-2 are shown to be responsible for complement activation through proteolytic cleavage of C2 and C4, whereas MASP-3 has been suggested to inhibit the generation of the C3 convertase. So far the role of the truncated proteases MAp44 and Map19 remain unknown. The monoclonal antibody 4H2A9 is suitable for western blotting and immunoprecipitation experiments. Moreover, the antibody can be used in immunoassays detecting MASP-1, MASP-3 and/or MAp44.