Binds and activates the enhancer (delta-A element) of the CD3-delta gene. Functions in the specification and the maturation of the T-lymphocyte. Also interacts with a critical control element in the TDT (terminal deoxynucleotidyltransferase) promoter as well as with the promoters for other genes expressed during early stages of B- and T-cell development. Function is isoform-specific and is modulated by dominant-negative inactive isoforms (By similarity). A number of isoforms are produced. Expressed in embryonic hematopoietic organs such as the bursa of Fabricius, thymus and spleen. In the adult, expressed in spleen, thymus, bursa and peripheral blood leukocytes. Expressed early in embryo from embryonic day 2 onwards. Belongs to the Ikaros C2H2-type zinc-finger protein family.