Updated project metadata. UFMylation, is a Ubiquitin-like modification occurring on RPL26, a ribosomal subunit, upon ribosome stalling at the ER membrane. This modification is achieved by a pool of enzymes commonly referred to as E1-activating, E2-conjugating and E3-ligase enzymes. The UFM1 E3 ligase comprises of three proteins: UFL1, UFBP1 and CDK5RAP3, referred to as UFM1 Ribosome E3 Ligase (UREL) complex. While UFL1 and UFBP1 are sufficient for E3 ligase activity in vitro, CDK5RAP3 is indispensable for ribosome UFMylation in vivo. In this work, we characterize in vitro UFMylation reaction of purified 60S ribosomes in the presence and absence of CDK5RAP3 using LC-MS/MS. Specifically, we analyse sites of UFMylation, linkage type and quantify monoUFMylation and diUFMylation on RPL26. Interestingly, we find UFMylation to occur on a specific lysine residue, K134, in the presence of UFL1/UFBP1 alone and in complex with CDK5RAP3. In addition, we also provide quantitative information on relative abundances of RPL26 mono- and di-UFMylation under these conditions.