Receptor (RTK) and nonreceptor (NTK) protein tyrosine kinases play a central role in trans- duction of extracellular signals, both in normal and neoplastic cells. Therefore, several approaches to inhibit PTKs have been developed. According to the structure of natural protein kinase inhibitors (quer- cetin, erbstatin ) Levitzki and coworkers prepared first synthetic tyrosine kinase inhibitors and coined the term „tyrphostins” (tyrosine phosphorylation inhibitors). Over 30 tyrphostins are now in various stages of clinical development.