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I would suggest moving to GMP as early as possible, potentially even during R&D work. This is especially the case if you are using cytokines or growth factors from a supplier that doesn’t also offer GMP for further manufacturing. Proteins are made in biological systems and are, therefore, susceptible to variability. Growth factors from different suppliers can appear identical on a Certificate of Analysis (C of A), yet still exhibit differences in folding, glycosylation, or impurities that can impact activity in your system. The earlier you incorporate the appropriate materials into your process, the easier and less expensive it is to make changes if they become necessary. Often cost is the main reason for delay. In order to keep costs down, work with your supplier to develop supply agreements that take into account bulk purchases, or bundling of other reagents and instrumentation.