For instance, a mutant of c-Src, in which Y527 is replaced by phenylalanine, is transforming and displays 5 to 10-fold elevated kinase activity compared to its normal counterpart. Csk has been identified as a Src related tyrosine kinase having both SH2 and SH3 domains and a catalytic domain but lacking sequences amino terminal to the SH3 domain as well as the carboxy-terminal regulatory sequences. Csk phosphorylates Src on Y527 and also down regulates Lyn, Fyn and Lck by tyrosine phosphorylation of carboxy-terminal regulatory sites. A Csk-like protein- tyrosine kinase of mouse origin (Ctk), also designated Ntk, and its human homolog, Lsk, have also been described.