To cope with the steadily growing body of knowledge and for organizational reasons the articles in this book are focussed on the modifiers ubiquitin, SUMO1/2/3, NED 8, ISG15 and FAT10 but it should be at least mentioned here that enzymes involved in conjugation and deconjugation of URM1, UFM1, ATG8 and ATG12 have been reported and continue to be investigated.