Hirschsprung disease (HD) has been described in association with microcephaly, mental retardation and characteristic facial features, delineating a syndrome possibly caused by mutations localized at chromosome 2q22–q23. We have analyzed a de novo translocation breakpoint at 2q22 in one patient presenting with this syndrome, and identified a gene, SIP1, which is disrupted by this chromosomal rearrangement. SIP1 encodes Smad interacting protein 1, a new member of the δEF1/Zfh-1 family of two-handed zinc finger/homeodomain transcription factors. We determined the genomic structure and expression of the human SIP1 gene. Further analysis of four independent patients showed that SIP1 is altered by heterozygous frameshift mutations causing early truncation of the protein. SIP1, among other functions, seems to play crucial roles in normal embryonic development of neural structures and neural crest. Its deficiency, in altering function of the TGFβ/BMP/Smad-mediated signalling cascade, is consistent with some of the dysmorphic features observed in this syndrome, in particular the enteric nervous system defect that underlies HD.