A novel protein-tyrosine kinase, designated 'Focal Adhesion Kinase' (FAK), has recently been implicated in signal transduction pathways activated by extracellular adhesion molecules and by neuropeptide growth factors. Previously deduced primary structures for chicken and mouse FAK polypeptides differ at their amino-termini, with mouse FAK reported to have a 25 amino acid residue extension not present in chicken FAK. Additional sequence information from the 5'-end region of the chicken FAK transcript now indicates that the amino-terminal extension previously thought to be unique to mouse FAK is, in fact, also predicted for chicken FAK. Thus mouse and chicken FAK polypeptides appear to be structurally similar throughout their lengths. This is further supported by comparison of their electrophoretic mobilities.