The vast majority of discussions we have about animal-free vs animal-sourced growth factors involve the risk of introducing pathogens. However, there are other points of variability that could be introduced by using animal-derived materials. “Animal-sourced” might mean that the natural growth factor is purified from a preparation like plasma. It may also refer to recombinant proteins produced in mammalian cell lines, or that the manufacturing process uses animal-derived components like serum. Natural proteins typically have more inherent variability than recombinant proteins, which are manufactured from scratch in a more controlled environment. For recombinant proteins, host cell protein is a potential contaminant that could affect your cells. Typically, host cell proteins would be found at very low, or undetectable, concentrations and the risk is small. However, if your cell type is exquisitely sensitive, it could have an impact. Lastly, growth factors produced in eukaryotic cells may be glycosylated, a post-translational modification that can affect activity. Consistent manufacturing conditions should lead to consistent glycan patterning, although anytime you rely on biology there is an inherent risk of variability.