The THAP domain includes a C2CH signature, an AVPTIF box, and several other conserved amino acids. It appears to be restricted to animal proteins and shows similarity to a DNA-binding domain of Drosophila P element transposase. The deduced 314-amino acid THAP11 protein contains an N-terminal THAP domain, followed by a central 30-amino acid polyglutamine tract. Ronin was expressed primarily during the earliest stages of embryonic development, and that its deficiency produced periimplantational lethality and defects in the inner cell mass. Conditional knockout of Ronin prevented growth of embryonic stem (ES) cells, while forced expression of Ronin allowed ES cells to proliferate without differentiation under conditions that normally would not promote self-renewal.