To assess whether, in principle, JOSD2 and ATXN3L also have serine esterase activity, we chemoenzymatically prepared the simple ubiquitinated serine substrate (Ub-Ser) in sufficient yield for comparison of all MJD members using the MALDI-TOF assay (SI Appendix, Fig. S15). Here, JOSD2 is linked to Spinocerebellar ataxia type 3.