CUL7 binds to SKP1, RBX1, and FBXW8 to form a cullin-RING ligase, or an SKP1-cullin-F box protein complex. The targeted disruption of the Cul7 gene in mice results in significant reduction in embryo size and neonatal lethality. In humans, CUL7 was found to be mutated in the 3-M dwarfism syndrome characterized by severe pre-and postnatal growth retardation, indicating that CUL7 is closely associated with human and mouse growth. We generated mice lacking Fbxw8 by gene trapping. Similar to Cul7−/− animals, Fbxw8−/− embryos and placentas were smaller than wild-type and heterozygous littermates and placentas. Approximately 30% of the expected number of Fbxw8−/− mice survived birth, but these mice remained smaller than their wild-type and heterozygous littermates throughout postnatal development. FBXW8 expression was detected in most organs of wild-type mice examined, and the organs in Fbxw8−/− mice were smaller than those in wild-type mice. Fbxw8 expression levels were highest in skeletal muscle, cartilage, and lung tissue. Expression profiling revealed elevated levels of insulin-like growth factor binding protein 1 (IGFBP1) transcripts in Fbxw8−/− embryos. Furthermore, we observed increased levels of IGFBP2 in Cul7−/− as well as Fbxw8−/− fibroblasts. These results demonstrate that the FBXW8-CUL7 complex plays a significant role in growth control.