MAp44 is a ~44 kDa alternate splice product of MASP1 and is mainly expressed in the heart. Mannose/mannan binding lectin (MBL) associated serine proteases, MASP-1 and MASP-3 are other products of MASP1. Similar to MASP-1 (isoform 1 of MASP1, which represents the longest transcript), MAp44 has a C1r/C1s/Uegf/bmp1 (CUB) domain, calcium-binding EGF-like domain and complement control protein (CCP) domains. However, it lacks the serine protease domain of MASP-1 and therefore cannot perform MASP-1's functions. MAp44 binds to multimeric pathogen receptors such asMBL and the three ficolins, and is believed to play a regulatory role in the lectin pathway of complement activation.