We have cloned a novel human protein tyrosine kinase gene specific to T cells by the polymerase chain reaction method. This gene encodes a 620 amino-acid polypeptide including a catalytic domain for tyrosine kinase, an SH2 domain and an SH3 domain, seemingly belonging to the src family. However, characteristics of a long unique N-terminal stretch and lack of a myristylation site at the N-terminus and of a kinase regulatory tyrosine residue in the C-terminus classify this molecule into a new subfamily comprising recently cloned mouse tec, itk/tsk and human atk/bpk genes. This gene was transcriptionally induced in normal T cells by interleukin 2 stimulation. These results suggest the crucial roles of this gene in T cell proliferation and differentiation.