A partial murine gene homologous to known mammalian sialidases reported in the Celera database is shown to encode a novel neuraminidase, designated as murine neuraminidase 4 (Neu4). The complete gene contains four exons and an open reading frame of 501 amino acids. It shows significant homology to the previously cloned neuraminidases with characteristic conserved motifs ascribed to the neuraminidase active site. Of the other known murine sialidases, it is most similar to Neu3 (42%). A cDNA encoding the entire coding sequence was isolated from mouse brain and expressed as a recombinant protein in COS-7 cells. Cell lysates contained significantly increased sialidase activity over mock transfected cells, demonstrating that Neu4 is a bona fide neuraminidase.