Using polymerase chain reaction–coupled subtractive hybridization, we have isolated genes expressed during the in vitro differentiation of murine embryonic stem cells into endothelial cells (ECs). Among the genes obtained, we identified one gene that was inducible by vascular endothelial growth factor (VEGF) in the murine EC line MSS31. Analysis of the nucleotide and deduced amino acid sequences revealed that the protein was composed of 930 amino acids, including an HEXXH(X)18E consensus sequence of the M1 aminopeptidase family, and is thought to be a mouse orthologue of puromycin-insensitive leucyl-specific aminopeptidase (mPILSAP). The recombinant protein hydrolyzed N-terminal leucyl and methionyl residues from synthetic substrates. Immunohistochemical analysis revealed that mPILSAP was expressed in ECs during postnatal angiogenesis. Specific elimination of mPILSAP expression by antisense oligodeoxynucleotide (AS-ODN) attenuated VEGF-stimulated proliferation, migration, and network formation of ECs in vitro. Moreover, AS-ODN to mPILSAP inhibited angiogenesis in vivo. These results suggest a novel function of mPILSAP, which is expressed in ECs and plays an important role in angiogenesis.