A novel STE20-like protein kinase, Mess1, was cloned from a murine liver cDNA library. The 1.7 kb cDNA encodes a peptide of 497 amino acids. Mess1 is most similar to human MST2 protein kinase with an identity of 95%. A putative kinase catalytic domain, located at the amino terminus of the Mess1 protein, is homologous to that of the STE20 family. There is a serine/threonine and glutamic acid-rich cluster in the carboxyl terminal region of Mess1 that is believed to mediate the binding of SH2 domains. Mess1 might involve the signal transduction on the interaction with proteins with SH2 domains.