Using a polymerase chain reaction based cloning strategy and conventional cDNA library screening we have characterized a novel intracellular protein tyrosine kinase from mouse mammary tissue. The gene encodes a protein with structural characteristics reminiscent of the src family of protein tyrosine kinases, a partial myristylation signal with glycine in position +2, SH3 and SH2 domains followed by a C-terminal tyrosine kinase domain. Interestingly, a potential bipartite nuclear localizing signal is located within the SH2 domain. A data base search revealed the highest homology to the human frk/rak gene. Northern blot analysis of mammary gland development indicated preferential epithelial expression in resting mammary glands and up-regulation in mammary tumours. Expression in organs was highest in the intestinal tract and was confined to the small intestine. We have named this gene iyk, (intestine tyrosine kinase), reflecting its pattern of expression.