Protein kinases play a key role in cell growth regulation. We have isolated a cDNA fragment of the MST gene from the MKN28 gastric cancer cell line cDNA pool by degenerate polymerase chain reaction. MST-cDNAs were cloned from the human brain cDNA library. Nucleotide sequence analysis indicated that the MST gene encodes a novel putative non-receptor type of serine/threonine kinase with Src homology 3 (SH3) domain, two leucine zipper domains and proline rich domain. The deduced amino acid sequence corresponding to a part of kinase domain and leucine zipper domains of MST (amino acid codons 244-461) is almost identical to the published partial amino acid sequence of MLK2. MST is the first non-receptor type of serine/threonine kinase containing SH3 domain, leucine zipper domain and proline rich domain other than PTK1/Sprk. The MST gene was moderately expressed in brain, skeletal muscle and testis as a 3.8 kb mRNA, and the MST gene has been mapped to human chromosome 19q13.1-q13.2.