AbstractThe Zfp36l1 gene encodes a zinc finger‐containing mRNA binding protein implicated in the posttranscriptional control of gene expression. Mouse embryos homozygous for a targeted mutation in the Zfp36l1 locus died mid‐gestation and exhibited extraembryonic and intraembryonic vascular abnormalities and heart defects. In the developing placenta, there was a failure of the extraembryonic mesoderm to invaginate the trophoblast layer. The phenotype was associated with an elevated expression of vascular endothelial growth factor (VEGF)‐A in the embryos and in embryonic fibroblasts cultured under conditions of both normoxia and hypoxia. VEGF‐A overproduction by embryonic fibroblasts was not a consequence of changes in Vegf‐a mRNA stability; instead, we observed enhanced association with polyribosomes, suggesting Zfp36l1 influences translational regulation. These data implicate Zfp36l1as a negative regulator of Vegf‐a gene activity during development. Developmental Dynamics 235:3144–3155, 2006. © 2006 Wiley‐Liss, Inc.