We present the entire sequence of the mouse Fat orthologue (mFat1), a protein of 4,588 amino acids with 34 cadherin repeats, 27 potential N-glycosylation sites, five EGF repeats and a laminin A G-motif in its extracellular domain. A single transmembrane region is followed by a cytoplasmic domain containing putative catenin-binding sequences. mFat1 shows high homology to human FAT and lesser homology to Drosophila Fat. The sequence of this giant cadherin suggests that it is unlikely to have a homophilic adhesive function, but may mediate heterophilic adhesion or play a signalling role. Expression analysis shows that the mfat1 gene is expressed early in pre-implantation mouse development, at the compact eight cell stage. Whole-mount and section in situ analyses show that transcripts are widely expressed throughout post-implantation development, most notably in the limb buds, branchial arches, forming somites, and in particular in the proliferating ventricular zones in the brain, being down-regulated as cells cease dividing. RT-PCR detects widespread expression in the adult suggesting a role in proliferation and differentiation of many tissues and cell types.