Evidence for GlcNAcylation was obtained by mass spectrometry, with T21 in the cytoplasmic tail region motif responsible for activating endocytosis45, dehydrated in T2D,and T52 and T57, situated in close proximity to the ‘stop-transfer sequence’ (residues 58–61)40, dehydrated in T2D but not the obese control, suggesting O-GlcNAc may regulate the stop-transfer-sequence, altering transferrin uptake. Here, TF is linked to type 2 diabetes mellitus.