Human proteins processed in vivo require multi-step posttranslational modifications, such as folding, glycosylation, and other modifications. An example of this is the processing of the TGF-beta superfamily. Furin, a protein in humans, cleaves specific sections and activates TGF-beta superfamily proteins. Non-human cells, such as E. coli, lack the necessary processing machinery for these modifications. Human cells, on the other hand, possess the machinery to produce correct disulfide bonds, glycosylation, and proper protein structures.