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MST4, also known as mammalian STE2-like protein kinase 4, is a novel member of the germinal center kinase subfamily of human Ste2-like kinases and is closely related to MST3. The 416 amino acid full-length MST4 contains a C-terminal regulatory domain and an N-terminal kinase domain, both of which are required for full activation of the kinase. MST4 is highly expressed in the placenta, thymus, and peripheral blood leukocytes. MST4 specifically activates ERK but not JNK or p38 MAPK in transiently transfected cells or stable cell lines, and thus is biologically active in the activation of the MEK/ERK pathway mediating cell growth and transformation. Further, MST4 kinase activity is stimulated significantly by epidermal growth factor receptor (EGFR) ligands, which are known to promote the growth of certain cancer cells. Accordingly, MST4 has a potential role in signal transduction pathways involved in cancer progression. Three alternatively spliced isoforms of MST4 have been isolated, and isoform 3 lacks an exon encoding kinase domain and may function as a dominant-negative regulator of the MST4 kinase.