The first extracellular domain of the cadherins has been shown to exhibit extensive sequence homology with the amino termini of the HA1 chains of influenza strain A hemagglutinins. These regions of homology are known to be functionally important in both the cadherins and the hemagglutinins. The homologous regions harbor the tripeptide HAV, which has been identified as being the cadherin cell adhesion recognition sequence. Here we report that members of the rapidly expanding family of fibroblast growth factor receptors also possess HAV-containing regions. These regions are homologous to the HAV-containing regions present within both the hemagglutinins and the cadherins and appear to be involved in regulating the function of the fibroblast growth factor receptors. We speculate that the HAV motif may represent an evolutionarily conserved amino acid sequence that will prove to be functionally important in a wide variety of proteins.