Using the method of gene targeting in mouse embryonic stem cells, regulatory function of δEF1, a zinc finger and homeodomain-containing transcription factor, was investigated in vivo by generating the δEF1 mutant mice. The mutated allele of δEF1 produced a truncated form of the δEF1 protein lacking a zinc finger cluster proximal to COOH terminus. The homozygous δEF1 mutant mice had poorly developed thymi with no distinction of cortex and medulla. Analysis of the mutant thymocyte showed reduction of the total cell number by two orders of magnitude accompanying the impaired thymocyte development. The early stage intrathymic c-kit⁺ T precursor cells were largely depleted. The following thymocyte development also seemed to be affected as assessed by the distorted composition of CD4- or CD8-expressing cells. The mutant thymocyte showed elevated α4 integrin expression, which might be related to the T cell defect in the mutant mice. In the peripheral lymph node tissue of the mutant mice, the CD4⁻CD8⁺ single positive cells were significantly reduced relative to CD4⁺CD8⁻ single positive cells. In contrast to T cells, other hematopoietic lineages appeared to be normal. The data indicated that δEF1 is involved in regulation of T cell development at multiple stages.