We identified a novel cDNA encoding truncated ZAP-70, which lacked the SH2 domain and a part of interdomain B, and named it truncated ZAP kinase (TZK). TZK was expressed in the thymus, spleen, and lymph nodes with ZAP-70. TZK was expressed in CD44(+)CD25(-) thymocytes up to mature T cells, but ZAP-70 was not expressed in CD44(+)CD25(-) or CD44(+)CD25(+) thymocytes. ZAP-70 or TZK was transfected into P116 cells derived from a Jurkat T-cell line deficient in ZAP-70. The P116 cells with ZAP-70 induced the T-cell receptor-mediated signal transduction, but the cells expressing TZK did not. While ZAP-70 was accumulated at the immune synapse, TZK was not. Meanwhile, impaired phosphorylation of SLP-76, one of the substrates of ZAP-70, in P116 cells upon pervanadate stimulation was rescued in the cells expressing TZK. These findings show that TZK is a novel isoform of ZAP-70, which is expressed in pre-T-cell receptor-minus thymocytes and functions as a kinase not associated with T-cell receptor. (C) 2004 Elsevier Inc. All rights reserved.