Whereas animals deficient for either Sulf1 or Sulf2 alone have normal birth weight, full viability, and histologically normal organs, mice simultaneously deficient for both Sulf1 and Sulf2 exhibit low birth weight, ∼50% neonatal lethality, ∼80% lethality as of weaning, failure to thrive, and a variety of skeletal and renal defects that arise during late embryogenesis. This evidence concerns the gene SULF1 and Failure to thrive.