Using computer-based, motif-trap screening, we have identified a third member of the mammalian fat family, fat3. Human and rat fat3 are also similar to the Drosophila tumor suppressor gene fat. The rat fat3 gene encodes a large protein of 4555 amino acids with 34 cadherin domains, 4 epidermal growth factor (EGF)-like motifs, a laminin A-G motif, and a cytoplasmic domain. Each member of the fat family is differentially expressed in the central nervous system during development. While both fat3 mRNA and fat1 mRNA are abundantly expressed in the fetal brain, the expression of MEGF1/fat2 mRNA is restricted to the postnatal cerebellum. fat3 mRNA and protein expression in the brain peaks at E15 during embryonic development. During this time, robust fat3 immunoreactivity is also observed in the spinal cord. These data suggest that the fat3 protein plays an important role in axon fasciculation and modulation of the extracellular space surrounding axons during embryonic development. (C) 2002 Elsevier Science (USA).