Rek (retina-expressed kinase) has been identified as a putative novel receptor-type tyrosine kinase of the Axl/ Tyro3 family with a potential role in neural cell development. rek clones were isolated from a chick embryonic brain cDNA library with a DNA probe obtained by reverse transcriptase-polymerase chain reaction of mRNA from Mü ller glia-like cells cultured from chick embryonic retina. Sequence analysis indicated that Rek is a protein of 873 amino acids with an extracellular region composed of two immunoglobulin-like domains followed by two fibronectin type III domains with eight predicted N-glycosylation sites. Two consensus src homology 2 domain binding sites are present in the cytoplasmic domain, suggesting that Rek activates several signal transduction pathways. Northern analysis of rek mRNA revealed a 5.5-kilobase transcript in chick brain, retina, and kidney and in primary cultures of retinal Mü ller glia-like cells. Rek protein was identified by immunoprecipitation and immunoblotting as a 140-kDa protein expressed in the chick retina at embryonic days 6 -13, which corresponded to the major period of neuronal and glial differentiation. Transfection of rek cDNA into COS cells resulted in transient expression of a putative precursor of 106 kDa that autophosphorylated in immune complex protein kinase assays. Overexpression of rek cDNA in mouse NIH3T3 fibroblasts resulted in activation of the 140-kDa rek kinase and induction of morphologically transformed foci. These properties indicated that Rek has oncogenic potential when overexpressed, but its normal function is likely to be related to cell-cell recognition events governing the differentiation or proliferation of neural cells.