In-vitro analysis of GalNAc-T isoform substrate specifities. (A) One hundred and eighty-one peptide substrates from the SimpleCell data set were tested by in-vitro enzyme assays with recombinant GalNAc-Ts. Seventy-three peptides were not glycosylated by either GalNAc-T1, T2, or T3. When testing additional five enzymes (GalNAc-T5, T11, T12, T14, and T16), only five more peptides were glycosylated. (B) Number of unique peptide substrates for each GalNAc-T isoform.