Ste20 is a S. cerevisiae Ser/Thr protein kinase that functions upstream of the MAP kinase module. Mammalian and yeast homologs of this kinase are divided into two classes based on their structure and regulation. Members of the first class (Ste20, Cla4, and p21-activated protein kinase) contain a C-terminal kinase domain, an N-terminal regulatory domain and a small GTPase Rac1/Cdc42 binding domain. Members of the second class lack the GTPase binding sites, but are similar to the former class throughout the catalytic domain. This second class includes germinal center kinase (GCK), HPK, KHS, KRS1 and 2, MST1, 2, and 3, NIK, SOK-1, and TNIK. Traf2- and NCK-interacting kinase (TNIK) is most homologous to another NCK-interacting kinase, NIK. TNIK contains an N- terminal kinase domain, and a C-terminal GCK homology (GCKH) domain. The mRNA of TNIK is expressed highest in heart, brain, and skeletal muscle. Overexpression of TNIK activates the JNK pathway, and leads to the disruption of F-actin structures and the inhibition of cell spreading. In vitro, TNIK can phosphorylate gelsolin. Thus, TNIK is a GCK family kinase that may regulate both the JNK pathway and cytoskeletal dynamics.