IK1, the splicing isoform 1 of the transcription factor Ikaros (IKZF1), binds to the promoter of the PTCRA gene, encoding pre-TCR-alpha. IK1-bindining sites overlap with RBPJ-binding sites. Therefore, IK1 competes with NOTCH3 coactivator complexes for binding to the PTCRA promoter (Bellavia et al. 2007). It is unknown which IK1 heterodimerization partners are involved in binding to the PTCRA promoter.