Replacement of the membrane-distal residues R22, R29 or R32 by alanine and also the R39W (known to cause a mucolipidosis type II/III-like phenotype in humans (Ain et al, 2021)) and R39A mutations had no impact on LYSET protein levels following transient transfection (Figs. EV1F and 6C). Here, LYSET is linked to mucolipidosis type II.