The complete nucleotide sequence of the cDNA indicated that it was related most closely to EMT, a tyrosine kinase of T cells, and to the B-cell tyrosine kinase BTK, which is mutated in X-linked agammaglobulinemia in humans and X-linked immunodeficiency disease (XID) in mice. Other members of this gene family include BMX and TEC.

TXK, like BTK, is a member of the TEC subfamily of SRC-type (nonreceptor) tyrosine kinases. Like other members of the TEC subfamily, and unlike other SRC kinases, TXK lacks both the N-terminal myristylation signal and the C-terminal regulatory tyrosine. TXK expression is detected primarily in T cells and some myeloid cell lines, but not in a number of other cell types.