ARL6IP1 and hereditary spastic paraplegia: While our Rtnl1 and Arl6IP1 models of HSP represent very rare forms of disease, (identified in just 4 and 1 families respectively14,15), given that they are members of the larger ER-shaping group of proteins they likely also inform about the more common forms of HSP due to loss of Spastin, Atlastin-1 or REEP1.