The pivotal role that protein-tyrosine kinases (PTKs) play in the growth regulation of eukaryotic cells is manifest in the frequent appearance of members of the PTK family as growth factor receptors or as the transforming agents of acutely transforming retroviruses. A feature common to all members of the PTK family is a highly conserved catalytic domain which is characteristic of the group as a whole and whose activity appears to be tightly regulated within the cell by other domains of the PTK. Degenerate oligonucleotide probes corresponding to two invariant amino acid sequence motifs within the catalytic domains of all PTK family members were synthesized and employed in the polymerase chain reaction (PCR) to amplify cDNA sequences between them. An M13 PCR library was produced in this way from cDNA prepared against mRNA from the murine hemopoietic cell line FDC-P1. The PCR library was then screened by DNA sequencing for PTK-related sequences. Two sequences were identified that, on the basis of sequence comparison with known PTKs, may encode representatives of a new class of PTK.
