Mouse asporin protein and nucleic acid sequences are disclosed. The protein contains a unique aspartic acid region near the N-terminus. The central domain contains ten leucine rich repeats. Sequences consistent with other class I small leucine rich repeat proteoglycans (SLRP) are also observed. Methods of use for the protein include regulating the complement system, inhibiting fibrosis formation, regulating the growth of endothelial cells and angiogenesis, regulating or inhibiting the growth of cancer cells, and regulating the functions of neuromuscular junctions.