D-Bifunctional Protein Deficiency is an inherited neurological disorder characterized by weak muscle tone and seizures in newborns. Most infants are unable to learn developmental skills such as how to crawl or walk, and those that do may eventually lose these skills. Other common features include vision and hearing loss, brain abnormalities and dysmorphic facial features. The majority of children with D-Bifunctional Protein Deficiency do not survive beyond two years, but cases older than 7.5 years have been reported. Those that do survive beyond the first year have severe intellectual disability. D-bifunctional protein deficiency is caused by pathogenic variants in the HSD17B4 gene.