AP4B1 and hereditary spastic paraplegia: Spastic paraplegia Type 47 (SPG47) is a rare HSP subtype caused by autosomal recessive mutations in the ubiquitously expressed Adaptor-related Protein Complex 4 Subunit Beta 1 (AP4B1) gene, leading to a significant decrease in AP4B1 protein levels.5,6 SPG47 causes initial low muscle tone in infants, which progresses to lower-limb spasticity and muscle weakness.