The Ikaros gene is an essential regulator in the development and homeostasis of the mouse lymphopoietic system. To study the role of the Ikaros gene in the human lymphopoietic system, we cloned and characterized human Ikaros cDNAs. In the human, as in the mouse, differential splicing of Ikaros primary transcripts generates a family of lymphoid-restricted zinc finger DNA binding proteins, highly conserved in sequence composition and relative expression to the mouse homologues. Expression of Ikaros isoforms is highly restricted to the lymphopoietic system and is particularly enriched in maturing thymocytes. The Ikaros gene maps at a syntenic locus located on the short arm of human chromosome 7 and on mouse chromosome 11 next to the epidermal growth factor receptor (Egfr). The high degree of conservation of the Ikaros gene at the genetic and expression levels strongly suggests that it plays a fundamental role in the ontogeny of the lymphopoietic system across species.