CSK is a cytoplasmic tyrosine kinase that has been shown to downregulate the tyrosine kinase activity of the c-src oncoprotein through tyrosine phosphorylation of the c-src carboxy terminus (1). Cell transformation by src oncoprotein is caused by several oncogenic mechanisms, which interfere with the carboxy terminal phosphorylation. The CSK could therefore potentially function as an anti-oncogene. CSK is ubiquitously expressed in human tissues as 2 mRNA species of 2.6 and 3.4 kb, although in some tissues and cell lines, only the larger mRNA is detected (2). A yeast 2-hybrid system has been used to identify proteins associated with CSK. The Src homology-3 (SH3) domain of CSK associates with a proline-rich region of PEP, a protein-tyrosine p