SMAD regulates gene expression by interacting with different classes of transcription factors including DNA-binding multi-zinc finger proteins. SIP1, for SMAD interacting protein 1, is a member of the delta-EF1/Zfh1 family of 2-handed zinc finger/homeodomain proteins. SIP1 contains a SMAD-binding domain, a homeodomain and two clusters of zinc fingers on the N- and C-termini. SIP1, also known as SMADIP1, ZFHX1B and ZEB2 (zinc finger E-box-binding protein 2), can be induced by TGF∫ treatment. SIP1 plays a crucial role in normal embryonic development of neural structures and the neural crest. The human SIP1 gene maps to chromosome 2q22. Mutations in the SIP1 gene cause a form of Hirschsprung disease (HSCR). Patients with SIP1 mutations show mental retardation, delayed motor development, epilepsy, microcephaly, distinct facial features and/or congenital heart disease—all symptoms of HSCR.