NEK2 is closely related in its catalytic domain to the serine/threonine protein kinase NIMA of Aspergillus nidulans which is required for entry into mitosis and may function in parallel to the universal mitotic inducer p34cdc2. Like NIMA, the Nek2 protein is almost undetectable during G1 but accumulates progressively throughout S, reaching maximal levels in late G2. These observations demonstrate that NEK2 resembles Aspergillus NIMA, both in its catalytic domain, and cell cycle-dependent expression. Recombinant NEK2 is active as a serine/threonine-specific protein kinase and may undergo autophosphorylation. Both human NEK2 and fungal NIMA phosphorylate a similar, but not identical, set of proteins and synthetic peptides. NEK2 is shown to be expressed most abundantly in adult testis and there is increasing evidence that NEK2 is abnormally expressed in a wide variety of human cancers. This antibody is specific to isoform 2 of NEK2.