The EGF family of receptors belongs to the tyrosine kinase receptor (TKR) family and plays an important role during embryonic and postnatal development and also in the progression of tumors. Her-2/neu/c-erbB-2, a member of the epidermal growth factor receptor family, can be cleaved into a soluble extra cellular domain (ECD) and a membrane-bound stub fragment. Her-2 ECD from a breast cancer cell line SKBR3 was immunopurified and analyzed with matrix-assisted laser desorption ionization (MALDI) and carboxyl terminal amino acid sequencing. A sequence within the juxtamembrane region (only 11 amino acid residues) (P) under bar AEQRdown arrowAS (P) under bar was identified most likely as a primary site of cleavage, (P) under barAdown arrowEQRAS (P) under bar as a minor site, that generate the ECD. The sites of cleavage are within the signature motif P/GX(5-7)P/G highly conserved in the EGF receptor family. (C) 2003 Elsevier Science (USA). All rights reserved.