AP4B1 and Spastic paraplegia: It follows that four types of AP‐4‐associated spastic paraplegia do exist, all of them with autosomal recessive inheritance pattern: spastic paraplegia 47 (SPG47)/AP4B1, spastic paraplegia 50 (SPG50)/AP4M1, spastic paraplegia 51 (SPG51)/AP4E1, and spastic paraplegia 52 (SPG52)/AP4S1.