To gain further insight into how ST6GAL1 expression is regulated that ultimately affects N-glycan sialylation in cancer, we examined an array of breast tumor cell lines (ER+ and TNBC) for ST6GAL1 expression on multiple levels of mRNA, protein, enzymatic activity, and by the abundance of the cognate enzymatic product, the α2,6-linked sialic acids on cell surface glycans (Fig. 2 panel A, B, C, D, E, respectively). This evidence concerns the gene ST6GAL1 and cancer.