(A) FOS are produced by the activity of two PNGase enzymes: one located in the ER, and the other in the cytosol. In the absence of iminosugar inhibitors FOS produced in the ER will be exported via a FOS transporter to the cytosol for degradation. The presence of terminal glucose residues on the A-arm of the glycan prevents export from the ER, leading to an increase in glucosylated FOS in the ER in the presence of iminosugars. Misfolded glycoproteins targeted for degradation through ERAD are trimmed by the enzymes ENGase and a cytosolic mannosidase. In the absence of iminosugars the resulting glycans enter the lysosome for further degradation; however, the presence of glucose residues prevents this leading to a build-up of glucosylated FOS in the cytosol. (B) Following isolation, purification and fluorescent labelling, total cellular FOS containing both the ER pool (1 in A) and cytosolic FOS pool (2 in A) can be analysed by NP-HPLC. Shown here are representative chromatograms of total cellular FOS from control HL60s cells (upper panel) and HSL60s cells treated with 1 mM NB-DNJ for 24 h (lower panel). The glucose capped glycans visible in the lower panel are indicative of α-glucosidase inhibition.