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). Expression mainly limited to thymus, spleen and pronephros. Very low expression in liver. No expression in testis, brain, eye and muscle. Expression begins at day 3-4 in the yolk sac and at day 5-6 in the embryo proper. Belongs to the Ikaros C2H2-type zinc-finger protein family.