We have isolated a murine protein kinase fragment from 10-day mouse embryo RNA, using a modification of the degenerate PCR technique (Libert et al. 1989; Wilks 1989). Subsequent screening of an 8.5-day mouse embryo cDNA library yielded cDNAs giving a complete open reading frame of 775 amino acids (Fig. 1A). An in-frame termination codon occurs in the cDNA 15 nucleotides 5' to the first ATG in the sequence. This methionine codon is efficiently utilized in a cell-free translation system (data not shown) and is highly likely to represent the translation start site in vivo. The encoded protein contains a putative protein kinase domain of 257 amino acids and includes all 11 conserved subdomains (I to XI) that are the characteristic hallmarks of the serine/threonine kinase family (Hanks et al. 1987). The positions of the conserved glycine-rich phosphate anchor motif (residues Gly 6~ Ala 6s) and the nearby invariant lysine (residue Lys 82) are indicated (Fig. 1A). In addition, the protein has a short N-terminal extension of 50 amino acids and an extended carboxyl terminal tail of 468 amino acids. There are no hydrophobic stretches characteristic of signal peptides or transmembrane subdomains, suggesting that the protein may have an intracellular function. The most closely related kinase sequences (6671% similar) in the literature are those of the kin 1 + gene from Schizosaccharomyces pombe (Levin and Bishop 1990) and the SNF, KIN1, and KIN2 genes from Saccharomyces cerevisiae (Celenza and Carlson 1986; Levin et al. 1987). These proteins comprise a distinct serine/threonine kinase subfamily (Hanks et al. 1987), with the kinl + and KIN1/KIN2 sequences further characterized by a large divergent carboxyl