Autosomal dominant spastic paraplegia type 42 is a pure form of hereditary spastic paraplegia characterized by slowly progressive spastic paraplegia of lower extremities with an age of onset ranging from childhood to adulthood and patients presenting with spastic gait, increased tendon reflexes in lower limbs, extensor plantar response, weakness and atrophy of lower limb muscles and, in rare cases, pes cavus. No abnormalities are noted on magnetic resonance imaging. The disease is hereditary spastic paraplegia 42 (Monarch Disease Ontology entry MONDO_0012928). Also known as: SLC33A1 autosomal dominant pure spastic paraplegia, SPG42, autosomal dominant pure spastic paraplegia caused by mutation in SLC33A1, autosomal dominant spastic paraplegia 42, autosomal dominant spastic paraplegia type 42, hereditary spastic paraplegia type 42.